Zitrone means lemon in German. Apparently I always look like I'm mad because people are constantly telling me to smile or asking me what's wrong. Anyway, my first year German teacher was this super-wacky guy (he'd keep a running count of how many rabbits he saw outside the window on the blackboard, write on himself with chalk, jabber at you in Russian if you spoke English without asking)--(sorry, I digress). Anyhow, he started calling me Die Zitrone (Die pronounced as Dee) because I always looked so sour.
Then I added 21 because it's my birthday (June 21, it's coming up and I'll be the big 25--eek!); so all my user names are some version of this: diezitrone, zitrone21, lemon21, etc, etc.