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?"
When a penis is placed under an armpit and the receiver moves there arm up and down like they are ringing a bell for a choo choo train.
This reminds me of my fourth grade camping trip when Mr. Hitchens broke into my tent, forced his cock under my arm, an ordered me to give him, an Armpit choo choo.
One must place ones cock inside anothers armpit and thrust persistantly untill ejaculation occurs.
Once I gave this top Lezza model an armpit choo choo, was so sweaty.