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A slight fluttering sensation felt in the flaccid penis as it starts becoming erect during normal everyday activity or while seated, usually as a reaction to some kind of unanticipated sexual stimulus. The trouser butterfly is also associated with a very slight movement of the penis as the erection progresses, making a noticeable movement through the trousers / pants.
Did you see that gorgeous new receptionist? She was so hot it gave me a trouser butterfly.

I got a trouser butterfly on the bus this morning because the seat was vibrating so much, and I'm sure the woman sitting opposite noticed.

Sandra asked me out for a drink at lunchtime - it gave me a trouser butterfly just thinking about it!

by Killdozer May 01, 2007
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