When a male unzips his pants fly and sticks his hand in his pants letting two fingers stick out of his fly. A female then lowers her vagina on the males fingers and together they move up and down stimulating the females vagina. (This may also be done between two girls, or a with a man being stimulated through his asshole. Usually referred to as other names like for a homosexual couple "Chocolate pronging")
Dude, did you Shanghai noodle that girl last night?
When a man sticks his index and middle fingers through his fly, then has a girl have sex with it as if it was his schlong.
Girl: Hey do you have two penises?
Guy: No, i'm just doing a Shanghai Noodle