Slang for spastic, mong and used towards someone with a lack of common sense or someone who has said or done something stupid. Usually preceeding the 'tongue in bottom lip' face; sometimes with a small, slow slap to said bottom lip and a 'duuuuuhhhhh' noise.
Can also be used for a not so clever hardman. A tough guy with a lack of intellect.
What did you go and say that for Bob, ya fucking Rembo. Duuuuuhhhhhh.
Did you see Mick chucking his weight around last night. Couldn't get his own way in an argument so went all rembo on the bloke.