Derived from two words: wang, meaning a penis, and face...well, duh.
So by calling someone a wang face, you're pretty much telling them they have the ugliest face you've ever seen.
It can often be used humorously as well, just because it sounds funny.
Hehe. wang. hehe.
also can be used to replace wanker.
mikerocks109: heyy you sexyyy, wanna go to a chat room to cyber?
xxkittykatxx: Ugh, no! wang face!
Tracy: Hahaha, did you see that wang face that was hitting on me this morning? What a fucktard!