Thumbs Up gesture has its origins in ancient Rome, and actually mean "Kill him,".In Rome crowd used to decide the fate of went down gladiator. either by shouting
Habet, Hoc habet! (He's had it!), Mitte! (Let him go!) or Iugula! (Kill him!). The gladiator would lay down his shield and raise his left hand to plea for mercy, which the crowd signified either by extending their thumbs up or down (pollice verso), turning the thumb upwards and jabbing it toward the heart (pollicem vertere). In Iraq Thumbs Up sign is a insult which means "Up yours, pal!"
source: Yahoo News