On two occasions in Parshas Behar the Torah instructs us not to afflict our fellow Jew. In the first instance, the Torah states: “When you sell an item to one of your people or buy from one of your people, a man should not afflict his brother.” A few passukim later, the Torah seemingly repeats itself: “Do not afflict your people and fear your G-d, because I am Hashem, Your G-d.” Chazal explain that there are two different types of onaah (affliction); the first passuk refers to onaas mammon - affliction relating to money. The second relates to onaas devarim - hurting someone through words. In general Chazal do not compare two specific mitzvos and say that one is greater than another, however, in this instance they compare the two forms of onaah. Initially, one would think that onaas mammon is more severe than onaas devarim because when a person is hurt verbally he does not lose any tangible object, however when he is afflicted financially then he suffers a real loss.
However, surprisingly, the Gemara says that onaas devarim is considered a greater sin than onaas mammon for three different reasons. Firstly, with regard to onaas devarim the passuk says, and you should fear your G-d” but it omits this when discussing onaas mammon. The Maharsha explains that people are more likely to notice when someone is trying to commit onaas mammon but that it is far easier to conceal one’s true intentions to harm people verbally. Someone who harms another financially is aware that people will likely recognize what he is doing but continues regardless. He shows a lack of yiras Hashem because he is unconcerned that Hashem is totally aware of his actions but he also demonstrates no fear of what people think of his actions. A person who harms people in a concealed way demonstrates that he fears people more than Hashem - he is only concerned that people not think he is a cruel person, but he is unconcerned that Hashem knows his true intentions. He is considered on a lower level than one who harms financially because he demonstrates greater concern for the opinion of other people than for Hashem.
Secondly, the Gemara says that onaas mammon merely harms people’s property, whereas onaas devarim is worse because it harms someone’s very being. This particularly refers to a person’s emotional well-being - the damage done to them by a careless word can penetrate to their very essence. A frightening example of this is related by Rav Dov Brezak Shlita: He relates how a well-respected Talmid Chacham in his forties required counseling because of a traumatic childhood experience - on one occasion his mother called him ‘tamay’. That single labeling damaged him so deeply that it stayed with him for the rest of his life. This provides ample indication that harmful words can cause untold damage.
The Gemara continues with a third aspect in which onaas devarim is worse than onaas mammon - if a person deceitfully extracts money from his fellow he can repair the damage by simply returning that which he unjustly took. However, when one harms someone else with words, no amount of apologizing can change the past - those words can never be taken back. It is a common occurrence in relationships, especially in marriage, that a few insensitive words have long-lasting damage and that damage can never be fully healed because those words can never be fully taken back. Perhaps a corollary of this aspect of the severity of onaas devarim is that once harmful words are spoken they can rapidly have a ‘domino effect’ whereby the consequences of these few words can be so far-reaching that it is impossible to ever undo the damage those words had done.
The following story, told over by Rav Dovid Kaplan Shlita, involves a situation where a few cruel words nearly had far-reaching consequences but they were averted by a few kind words: “Raised modern Orthodox, Devoras’s parents instilled in her a respect for rabbinic but a critical eye toward chareidim. When she got older, she decided to check it out for herself and davened at the Ponevezh Yeshiva during the Yamim Nora’im. She went back for Simchas Torah. Everything was fine until one of the girls present said to her in a loud voice in front of a crowd of girls, “you don’t come to daven here without wearing stockings!” Devora stormed out. If this was how chareidim behaved she was not interested. However, due to her respect for rabbinic, she decided to go speak to Rav Shach. When she arrived at his door, there was a long line of men waiting to go in. When the door opened and the person inside left, they called here in, explaining that women had higher priority. Pleasantly surprised, she related the shocking story to the gadol hador. “They did a big aveirah.” Rav Shach told her. “Maybe it was unintentional, but they are still obligated to ask your forgiveness.” He spoke to her for a long time about how careful we must be to be sensitive to others. She decided during this talk to become more religious. Today she is married to a Rosh Yeshiva and her sons and son-in-laws are talmidei chachamim.” This story teaches us how much damage one wrong statement can do - it caused this girl great pain and anger, and very nearly prevented her from becoming more observant. It also demonstrates how much good a few thoughtful words can do.
It is very clear from the Gemara how serious the sin of onaas devarim can be, howover it is a very difficult mitzvo to observe properly - we are constantly involved in conversation with other people and it is very easy to hurt their feelings through a thoughtless statement. Moreover, because we speak so much we can forget how serious a sin it is to hurt other people’s feelings. The Chazon Ish once witnessed a man strongly rebuke his young son for moving something on Shabbos that may have been muktza. The Chazon Ish told the man that his son may have transgressed a Rabbinical mitzvo, but that the father had definitely transgressed the Torah mitzvo of onaas devarim.
One technique to help be more watchful of this mitzvo is to develop the attitude that we should be just as careful in it as in all other mitzvos such as kashrus - we would never eat something without being certain that it was permitted to eat it. So too, we need to try to develop a sense of vigilance that what we are about to release from our mouth is permitted. The best way of doing this is by learning the halachos and hashkafo behind it.
It is instructive to end with one final saying of the Chazon Ish - he used to remark that one of the greatest possible sources of joy is that he lived his whole life without causing pain to his fellow Jew - may we all be zocheh to only do good with our speech..
 Behar, 25:14
 Behar, 25:18.
 Onaas mammon consists of deliberately selling an item for an extortionately high amount or deliberately buying an item for an exceedingly low price.
 Bava Metsia, 58b.
 Maharsha, Bava Metsia, 58b. He compares this to Chazal’s saying that a ganav is worse than a gazlan because the ganav steals in private because he is worried about people seeing him steal but has no concern that Hashem knows he is stealing, whereas a gazlan steals in public and shows equal disregard for people and Hashem.
 Kaplan, Major Impact, p.93-4.
 The Sefer Mishpatey Shalom, Ch 7 is a good source for the laws pertaining to onaas devarim.