Basically, it's a person bowing! The two M's are the hands of the person, the _'s are the person's eyes, and the ()'s are the sides of the person's head.
Person 1- I command you to do this!
Person 2- Yes, master. m(_ _)m
A refusal to answer a question because the response could hold self incriminating evidence.
Cop: We know you have drugs in this house. Where are they?!
You: Ask me anymore questions and I plead the 5th!