Mine comes from my father calling me Thin Lizzie (he called his cars Tin Lizzies) when I was a child, and my younger brothers using the Liz part because they found it easier to pronounce than my real name. I'm still Liz to both of them decades later even though it's not even a derivation of my real name.

I'm glad this thread was started since several usernames that I couldn't figure out have been explained.