ETYM Old Eng. thritty, AS. thrîtig, thrittig; akin to Dutch dertig, German dreissig, Icel. thrjâtîu, thrjâtigi, thrir teger, Goth. threis tigjus, i.e., three tens. Related to Three, and Ten, and cf. Thirteen.
Being ten more than twenty; SYN. 30, xxx.

The cardinal number that is the product of ten and three; SYN. 30, XXX.