A Thumb-Breaker is a more threatening version of a D.U.F.F.(designated ugly fat friend). Instead of just preventing you from talking to a girl a Thumb-Breaker (usually 250lbs+)will physically try to harm and or paralyse the persuer. Thumbs could be broken!
Hey man did you get her number yet? "Nah man her Fuckin Thumb-Breaker just kicked my ass in front of everybody!!"