This tickled me.
In a thread on the SDMB one poster insultingly invited another to kiss his taint. The insultee, as puzzled as I was by the term, asked its meaning. Another poster obliged with this definition:
The taint, I am given to understand, is the tender fold of flesh between the anus and the scrotum that is characterized chiefly by its seam, which forms a pinched ridge where one hemisphere of the body joins the other.
I hurried to the OED for confirmation but found no reference to this (presumably) scientific anatomical term.
Then I spotted this followup post and the lights went on!
T’ain’t your ass and t’ain’t your balls