This word first surfaced in Paisley, Scotland during a conversation while watching saw III. A joke was made about a child filming his mother in her bedroom from the closet in order to put the video on his private website.
The perpetrator of the act of mumping is known as a mumper while the person whos mother is being humped is known as the mumpee.
You're getting mumped!
You just got mumped!
Shut it or I'll mump you!
I'm going to mump your maw! (hump your gran)
You mumper! (mother f*cker)
You got one hell of a mumping last night!