A Kirst is someone, normally female, who is very clingy and needy. A Kirst will not allow her man to so much as look at another chick, even if that chick is a friend. Upon so much as looking at another chick, a Kirst will go into stupidity overdrive and insinuate that you are cheating, an asshole, etc. Kirsts are also prone to self-loathing and misanthropy. A Kirst will generally spend 3/4 of their day bringing themselves, and everyone around them, down.
Girl: You're cheating on me, aren't you?!
Guy: She's... been my friend for years...
Girl: OMG, I knew it!
Guy: Fuck, stop being such a Kirst. -_-