Fornication involving the use of an armpit where the male inserts his big veiny bastard of a cock into the females armpit, possibly sweaty but most of all preferably freshly shaven, or Western European style to avoid all chances of unwanted friction. Then, soon after the male inserts his Trouser Trout into her slippery pit, she goes on to make an up and down arm motion, something similar to a conductor pulling on the train horn, hence the Choo Choo, and helping to create all sorts of additional pleasure for the male, leading to a climax superior even to that of having sex with a young, feeble sheep.
Man 1: "Hey man, I'm looking for somethin' new in the bedroom, any suggestions?"
Man 2: "Isn't your girlfriend from Paris?"
Man 1: "Yea, why?"
Man 2: "Ever heard of the Armpit Choo Choo?"