A rude or ill-behaved guest at a Walt Disney theme park.
(From a pun on the slang word "tool" and the "Mousekatools" in the Mickey Mouse Clubhouse show.)
That dude on the shuttle bus who glared at me as he set his backpack on the only remaining seat? Or the guy who ran over my foot with his scooter and didn't apologize? Total mousekatools.
Someone who jumps from one relationship immediately into another one.
Serial monogamists can not stand to be alone and often suffer from vast commitment and insecurity issues.
Because they jump into relationships immediately after the previous one has ended, serial monogamists typically don't take the time to reflect on their behavior or why their previous relationships failed; thus, they end up making the same relationship mistakes over and over again.
Person 1: Damn, Dustin already has a new girlfriend?! It's only been two weeks since he broke up with his fiance! I think he's a sociopath.