https://www.google.com/amp/www.urbandic ... e&=trueA hole located in a partition in which one's penis is inserted, thus separating the participants and ensuring anonymity throughout the act of copulation or fellatio (or possibly a titty wank).
In higher class establishments, this hole will be fabricated with duck tape to prevent genital chaffing.
There are some other colorful definitions, but I think that sums it up well. And yes, most of the time it's in men's room at a truck stop...