Mine's from an old Armed Forces Reserve commercial. Don't know if anyone remembers it, but it this kid in a classroom approaching the blackboard to solve a problem, and he narrates something like "After coordinating a group of tanks, helicopters and ground troops in the Army Reserve, college was no problem for me" - they cut to the guy smiling at his work and the professor saying "Nice work, Mr. Montez".
So anyway, one day I called my friend up while he was apparently stoned/drunk and watching this commercial:
Me: Hey Seth, how's it going?
Seth: Mr. Montez!!!
Me: What?
Seth: Nice work, Mr. Montez!
Me: What the hell are you talking about?
Seth: From now on I'm going to call you Mr. Montez.
Me: Well, from now on I'm going to call you an asshole, how about that?
Seth: Hahahahahaha!
For some reason the nickname stuck and everyone started calling me Montez. It pissed me off at first but I eventually started to like it, and it seemed like a good online name.