A ménage à trois arrangement (ménage à trois literally translates from french as 'a household of three' and is used mostly to mean a threesome, thus the
slang term) in which two individuals, (generally females)in cahoots with
one another, get together and fuck another, usually unsuspecting male.
Often, but not exclusively, this is done at
birthday parties, especially an 18th birthday party (or whenever the person being taken to france reaches the local age of consent).
Most commonly this is done by 2 girlfriends who want to do something
extra-nice for a guy friend.