Also means big toe. From Lewis and Short: "To close down the thumb (premere) was a sign of approbation; to extend it (vertere, convertere; pollex infestus), a sign of disapprobation." Premo 'to press (down)' and verto 'to turn'. Or as we say today, thumbs up and or down. (Reminds me of a 7Up soda drink in India called Thums Up.