As a Sci-Fi Geek, I've found a great many useful non-human swear words get the point across without actually being obscene. In the interests of maintain a family forum (so to speak), I present these alternatives to the unacceptable Sailor-talk.
For example, FarScape is a fountain of inoffensive extraterrestrial expletives like Frell (verb, to copulate), Dren (noun, excrement), Hezmana (proper noun, the Hynerian Netherworld), Yotz (noun, no direct translation), and Mivonks (noun, Luxan male genitals). "What the yotz is wrong with you? Don't give me that dren! Answer the frelling question! Or get the Hezmana away from me before I kick you in the mivonks! Frell you!"
Before the Klingon Dictionary was published, frustrated klingons in the novels and fanfic tended to mutter "Khest!" the same way the French say "Merde!" The Dictionary gives no translations, but offers the following.
Epithets (Nasty names to call someone): petaQ, toDSaH, taHqeq, yIntagh, Qovpatlh
General Invective (to be shouted when hitting one's thumb with a hammer): QI'yaH, ghuy'cha' (also spelled ghay'cha'), Qu'vatlh (often shortened to va), baQa', and Hu'tegh.
Other good sources are DC Comics' alien bounty hunter Lobo ("You fraggin' bastitch!") and Alien Legion's Jugger Grimrod ("You rammin' bospor!")
The Hitch-Hiker's Guide to the Galaxy mentions some truly blistering bits of galactic gutter language, such as "Swut," "Turlingdrome" and most obscene of all, "Belgium."
The best, though, is Red Dwarf's universal swear word, Smeg. It has such versatility, able to be used as a noun ("What the smeg?" "You're such a smeg-head!"), a verb ("Smeg off!"), and an adjective/adverb. ("What's your smegging problem?")
In order to maintain the Forum's high standards of decorum, I propose that we all restrict our cussing to the alien tongues I've just mentioned. If any of you bospors don't like it, you can just smeg off and go frell yourselves, you tunding petaQpu'!😀