Swear words primarily must be offensive. It has to be a taboo word that one uses out of context from its actual denotation. Most of our current swear words are slowly becoming less taboo, meaning their status as swear words is diminishing. There are some notable exceptions, like "nigger" and "faggot" that are becoming more taboo. However, words like "fuck" and "bitch" are definitely decreasing in their taboo level. When my grandmother can say "I am so sick of that fucking asshole", then you know the impact of those words is not quite the same as what it used to be (of course, my grandmother also works at a bar...).
Because of the necessity of the taboo of a word, made up words are not usable as swear words. For new swear words, you must find a word that is not commonly used, is taboo, and is not currently used as some form of epiteth. Racial slurs are quite taboo, but they're already in use, so they do not qualify. For a new curse word to be any good, people must blush or even gasp when they hear it. This is not a nicey-nice game.
I mentioned that the word is used out of context. Words often have conflicting conotations and denotations. Take this (very) short list
word denotation conotation
fuck to copulate (when used alone) an expression of anger
bitch a female dog a complaining woman
shit fecal matter an expression of anger
faggot a bundle of a homosexual male
twigs used to
Notice that the words themselves have certain features in common, specifically they are short, often monosyllabic (the word "faggot" is most frequently used as just "fag"), and often composed of hard consonant sounds. Thusly, the word "languish" would not make a good curse word, as it is too long and composed of mainly soft sounds.
So, what short, harsh words fit this necessary taboo nature, and in what context would using them make them a reasonable curse word?