An ordinary word a particular character uses to swear with.
Lots of characters have favourite Catch Phrases -- this guy has one he swears by. Someone has a particular non-obscene word they like to use as an expletive. May be an example of Goshdang It To Heck if they don't like swearing "properly", or may just be a sign that they're a bit odd. A kind of Catch Phrase - if it's only used once, it's more likely an Unusual Euphemism. If the 'swearword' in question is used setting-wide, it's Pardon My Klingon - this trope is a particular character's idiosyncratic swearing. See These Tropes Should Watch Their Language.