The Prophet (peace and blessings of Allah be upon him) said, “The Muslim is he whom Muslims are safe from harm of his hand and tongue.” (Al-Bukhari 1/53, Muslim 1/65)
There are different narrations of this hadith which have people instead of Muslim and other has believers. This hadith means any harm that comes from the tongue this could be some of the following: Cursing, swearing, backbiting, slander and tale bearing. Swearing is a major sin it is not a minor sin because like other major sins it affects others as well as yourself.
Saying an offensive word affects the person who you said it, the one who heard it and the one who it was said to. It is not a major thing now that we hear so many people swearing all over the place.
Ibn Mas’ud (may Allah be pleased with him) states that the Messenger of Allah (May Allah bestow peace and blessings upon him) said, “A believer does not falsely claim lineage; does not curse; is not lewd or obscene.” (At-Tirmidhi 1987)
The Prophet (may Allah bestow peace and blessings upon him) was asked, "What is Islam?" He replied, "Gentle speech and feeding the poor." Imam Ahmed.
People also seem to think that it is okay to swear in another language and that somehow that does not count! It does count, swearing in any language counts and is counted. We do speak in dialects of the languages of our various peoples and they often contain rude, offensive and insulting language. This is extremely common sight that you may witness someone speaking in English then switching to his mother tongue to let rip on some people who do not understand him, then turn to them smiling! One of my English friends asked me what two people were saying and I told him that it was better off that he did not know. Why? Because they were swearing!
This is the main reason that we do not like the use of some dialects. They are littered with swearing and insulting language and if you want to join in the conversation then you have to join in on the sinning!
You see hearing is not something that you stop; even if you put your fingers in your ears you can still hear what someone says!
It is bad enough to swear at someone but swearing at a Muslim is even worse.
The Messenger of Allah (peace and blessings of Allah be upon him) said, “To insult a Muslim is corruption and to wage war on him is disbelief.” (Bukhari 48, Muslim 64)