It offends God and the angels. And pleases Satan and the devils. The former depart away the latter arrive.
Here is an extract from the writings of St. Jean Baptiste de la Salle on this subject.
'In an effort to get those who swear to stop this habit, Ecclesiasticus warns them that sorrow will never leave their home and that their house will always be filled with affliction (Eccl., 23:10–11]).
Following the Wise Man’s advice, therefore, take care not to have God’s name constantly on your lips, and do not use the names of the saints in conversation, even if you do this only thoughtlessly, from mere habit, and without any evil intention. You must not pronounce the name of God and of his saints irreverently and without good reason. It is never appropriate to mingle in your ordinary talk such words as “Jesus!”, “Mary!”, and “Oh, my God!” nothing, such as “By Jove!”, “What the devil!”, “By heaven!”, and others.
Words like these must never be found on the lips of a wellborn person. When you use such an expression in the presence of people toward whom you are obliged to show esteem, you offend against the respect due to them. According to the opinion of the Wise Man, you cannot excuse yourself on the pretext that when you swear, you harm no one, for this excuse, as he remarks, does not justify you before God (Eccl [Sir] 33:14 [23:9–14]).
You must limit yourself, as Jesus Christ advises you in the Gospel, to saying, “This is so,” or, “This is not so” (Mt 5:5 ). If you wish to add some emphasis to your words, it is sufficient to say, for example, “I assure you, sir, that this is how it is,” without adding anything more.
You must entertain no less horror for indecent words than for swearing, for they are no less contrary to civility and often are more dangerous. Saint Paul desires that the Christians of his day act on all occasions with decorum. He warns them in several places in his epistles to take special care lest any indecent word escape their lips. He expressly forbids them among themselves even to mention fornication (Eph 4:29). It shows a lack of respect to use dirty language. To amuse your companions, you must never use language that is even a little too free, not even under the pretext of joking and playfulness, for, says Saint Paul, if you wish to be agreeable to others when speaking, you must say something that can edify (Eph 5). In this matter, even doublemeaning words are not permitted, for you would offend against politeness as well as against propriety. The same can be said of words that suggest, even in the slightest degree, some indecent idea or image.
When you happen to be in a group where some of those present use language that is a little too free or that wounds decency even in a slight degree, you must be very careful not to laugh. If you can, pretend not to have heard; at the same time, try to turn the conversation in another direction. If you cannot do this, you must show by your seriousness and deep silence how distasteful you find this kind of talk.
It can be said that you make your real self known by the sort of language you use, for, as Jesus Christ declares, out of the abundance of the heart the mouth speaks (Mt 12:34). Using language that is filthy and that shocks people’s sense of decency is one way of letting them know how impure and corrupt you are.’
St. Jean Baptiste de la Salle
‘Some people seem to glory in displaying their irreligious spirit in what they say by mingling the words of Holy Scripture with profane things, by mocking and making fun of sacred things and religious practices, or by bragging about the sins and infamous deeds they have committed. These are the very people of whom the Wise Man says that their conversations are intolerable, because they make a game and a joke of sin itself (Eccl [Sir] 27:15 ).’
St. Jean Baptiste de la Salle