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The act of forcing ones thumb into a partners rectal passage, without the act being discussed or pre arranged.

There is a high chance that this act is will be made by a opportunist and stealthily approached.
"Hey Pash whats that over there?"
"oooooh my god what the fuck was that?"
"i just thrum boned you dude!!!"
by Titanium Ukrainian December 14, 2007
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