A small machine made of a rubber band, wooden sticks and a spool. The rubber band is put through the spool and the sticks are wound tightly on each end, creating tension. The user then sits on a wooden or metal chair with the fart harp under his butt. At the proper moment the "harper" leans to one side, allowing the fart harp to spin against the hard chair. This creates a loud farting noise.
He was thrown out of class for sounding a "fart harp" in the middle of the lecture.
A term made famous in a Jane Russell movie where she stands large- breasted between two cowboys. One cowboy says "You stole my horse so I stole your girl." The other cowboy answers "Yeah, tit for tat."
"I cut your grass and you wash my car, you know, "tit for tat"."