One's girlfriend or wife. Usually used as a casual term of affection. Popularized by fans of the band Ween, as used on the EZ Board BBS dedicated to Ween. Such users generally attribute Buttsled as the father of the term.
"Last night the yak and I didn't do much, just drank some beer and listed to The Pod."
"First originated in the small town of Danville, Indiana, this word was first used on the baseball team by Alex Mershon. This word describes a situation where you are at a lose of words to describe the amazing feat that has been laid out before your eyes. This is where the word yak is used. Sometimes when something is extra amazing the "a" part of yak can be drawn out to produce the word yaaaaaak"