When fucking doggystyle, stick your thumb up the girls ass, if she takes it willingly then she has passed the thumb test so its safe to put your cock in next!!
I gave this girl the thumb test last night and she past with flying colours.... well, maybe only 1 colour!
by The Alvinator March 9, 2011
A thumb test is when you stick your thumb in a girls mouth.. If she sucks on it, it means she wants your friend to join in. If she spits it out, it means she does not.
by Mr.CJackson January 25, 2011