What a fun thread! It's neat to see where people get their nics from. The origin of my username is kinda sappy, but here goes. One Sunday I was at church when I was still pregnant with my daughter, and an adorable little girl (maybe 3-4yo) came up to me in the hall:
: You have a big tummy like my mommy. She's gonna have a baby. Are you gonna have a baby, too?
: Yes, I am.
: [pausing, tilting her head and kind of looking at me like she's appraising my appearance] You're very pretty.
: awww, tha...
: [cutting in]...but now you'll be mommy
My heart just about melted. "Mommy pretty"—I loved it! A few days later I was setting up a new IM account, and I thought I'd try "mommypretty" as my username; that was taken (go figure!) so I tried "mommiepretty" instead—it worked, and I've been mommiepretty ever since (8+ years now)