a name used to substitute someone's real name so they don't know you are talking about them. It comes from the vietnamese word for diarrhea (lao sai)
Person 1: Dude did you see what Sai Lao did the other day?
Person 2 (Sai Lao): Who's Sai Lao?
Person 3: Yeah man! What a Dumbass!
Person 2: Who?!?