What would a theory of foul language look like?
1. Polysemy and plasticity. A word would typically cover a wide semantic range and syntactic function. Shit as expression of surprise or sympathy. As a stand-in for a generalized stuff, etc... As a synonym for abuse: "I'm not taking any more of your shit!" I would guess that it would have at least 100 separate lexical uses if we count compound words and idioms like "shit-eating grin." The f word dictionary I purchased is 267 pages. Swearing is not, as your mother said, the province of those with a small vocabulary, but is (instead) a wonderful manifestation of linguistic creativity.
2. Most uses will be metaphoric or hyperbolic. "Tastes like shit." It tastes bad, but not necessarily of literal shit. There may be a few "master metaphors" behind most of profanity. Excrement as debasement. Being fucked or screwed as being on the receiving end of bad treatment. You could be a splitter, trying to find as many separate lexical items as possible, or a lumper, trying to find those broad categories.
3. There will be a range of substitutes or euphemisms, displacement of the core word: Shoot. Crap. Load of bull. Miércoles for mierda. Effing, friggin, ferkin. There will be multiple terms for the same thing, many words for the same genitals, for example. La concha or la vaina.
4. There will be idiomatic compounds, of the "fucking asshole" variety. There will be a syntax of relations between words, and the way they relate to other words. Tmesis like "absofuckinlutely." Stand alone profanity. "Shit!." Adjectives, verbs, nouns, adverbs, etc...
5. Taboo language will be scatological, sexual, or theological. A fourth category might be the animal kingdom. Cabrón / bitch. The taboo language of ethnic and racial insults (wap, mick, etc...) is the subject of another post.
6. Reversibility. Something bad can be good: "... de puta madre" = wonderful. "Motherfucker" = someone with an amazing talent to play jazz. "That's some great shit."
7. It has a fundamentally affective use. In other words, it is designed to convey an attitude of emotion or aggression beyond the normal. Only it's taboo nature permits it to carry this affect. Yet the best swearing is done almost in passing, as though that were the normal way of saying something.
8. Aggression and solidarity. Those might be the two functions of profanity. Swearing with someone gives you a sense of belonging with that person. Swearing at a person is saying I can swear at you, because we have a level of familiarity. Or not. It can be a sign of aggression. Even as the expression of solidarity it is a kind of "intimate aggression"?
9. The non-metaphorical sexual use of sexual vocabulary is something a bit different. Say, two people in bed using that vocabulary to describe what they are doing to each other, or what they would like done.
All this is very serious. I am developing ideas to teach (again) this material. I know little about it, though I can swear competently in two languages. I am just brainstorming on the blog.