A bastardization of the word mustard, for people who like to garnish their burgers and dogs and swear at the same time. Goes well with katshit.
Joe: All this burger needs is a little mouseturd ... mebbe some katshit too.
Celia: <sigh> Can't you say anything without fucking swearing for once?!
Joe: <rolls his eyes> Tse, I'm just kidding, for fuck's sake!