Way back in the old BBS days, the time came for me to figure out a handle. (Screen name? What's one of them?). At the time, I was reading a book by Terry Pratchett called "Mort"... so I became Mort.
Fast forward to roughly 1996 when I got my first actual pop3 e-mail account. Mort was already taken, so I went with Mortimer.
Somewhere in that time-period, people in chat rooms started calling me Morti. It stuck thatway.