The history (ie precedent) of viewing crude language (whatever other synonyms or euphemisms we might use to describe the act) as sinful is quite undeniable. It is a venial sin, but one that, in the extreme, can actually be mortal.
See here
http://www.newadvent.org/cathen/04573d.htm
Last paragraph
Curses which imply rebellion against Divine Providence, or denial of His goodness or other attributes, such as curses of the weather, the winds, the world, the Christian Faith, are not generally grievous sins, because the full content and implication of such expressions is seldom realized by those who use them. The common imprecations against animate or inanimate objects which cause vexation or pain, those against enterprises which fail of success, so, too, the imprecations that spring from impatience, little outbreaks of anger over petty annoyances, and those spoken lightly, inconsiderately, under sudden impulse or in joke, are, as a rule, only venial sins — the evil being slight and not seriously desired. To call down moral evil upon a rational creature is always illicit, and the same holds good of physical evil, unless it be desired not as evil, but only in so far as it is good, for example, as a punishment for misdeeds, or a means to amendment, or an obstacle to commission of sin; for in such cases the principal intention, as St. Thomas says, is directed per se towards what is good. When, however, evil is wished another precisely because it is evil and with malice prepense, there is always sin, the gravity of which varies with the seriousness of the evil; if it be of considerable magnitude, the sin will be grievous, if of trifling character, the sin will be venial. It is to be noted that merely verbal curses, even without any desire of fulfilment, become grievous sins when uttered against and in the presence of those who are invested with special claims to reverence. A child, therefore, would sin grievously who should curse father, mother, or grandfather, or those who hold the place of parents in his regard, provided he does so to their very face, even though he does this merely with the lips and not with the heart. Such an act is a serious violation of the virtue of piety. Between other degrees of kindred verbal curses are forbidden only under pain of venial sin. To curse the devil is not of itself a sin; to curse the dead is not ordinarily a grievous sin, because no serious injury is done them, but to curse the saints or holy things, as the sacraments, is generally blasphemy, as their relation to God is generally perceived.
