Mine is one of the worst...
Crotchy
My sisters started calling me Katch or Katchy one year, no particular reason - maybe because I was a good catch?!?.
I was out with a sister one night and she introduced me as "Katchy, not to be confused with Crotchy!" About a week later I was walking on campus and I hear "CROTCHY!!!!" It was one of the guys I had met and he couldn't remember the actual nickname, so he yelled all he could remember. From then on, whenever I walked into a bar or party he was at, he'd yell real loud, "What up, Crotchy!"
