Anybody(mostly Bostonians) who say "er" at the end of words instead of "a" will call a pizza, a pizzer. Other examples are soder(soda), ideer(idea), and tuber(tuba). Anyway, zerman comes from a person who says the sentence(or something like it), "Hey, you wanna order a pizzer, man? "Zerman" being the last 2 syllables in the sentence. So, a zerman is a pizza.
Moke: I'm hungry.
Jimmy: Let's get that zerman.