Mystery Moose is the act of making a moose sound (and hand gesture on side of head) whilst performing a sexual act on a partner in the 'doggy style' position.
He was banging me from behind and I heard a strange moose sound, I asked him what this was and he said he couldn't hear it, I heard it again and again and he still said nothing. A short time later he started laughing to himself and revealed it was him making the sound, aka, Mystery Moose.
by Callum Reckless December 4, 2008