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The act of inserting an exploratory thumb into the anus of a girl whilst in the doggy position. If this is met with a positive reaction, or indeed no response, it can be taken as permission to change holes.
Friend 1: Did you shag her then?
Pioneer: Yeah, fucked her up the arse and everything. She was a right dirty slut.
Friend 1: Sound, how did you manage that?!
Pioneer: Well you know, just slipped in the pioneering thumb.
Friends 2 : Unbelievable tekkers.
by GenghisKorn October 03, 2010