What is The Big M?
1.
common term used by insecure teens trying to conceal talking about masturbating. Often used as a code word.
So dude I was doing the Big M and then my grandma walked in and scared the living jizz outta me.
See
2.
A subtle term for masturbation, used when you don't want to outright say that you are going to go masturbate.
"Be right back, I'm going to do a little of the big M."
See