An Etymological Dictionary of Astronomy and Astrophysics

A cardinal number, 10 times 3.

Etymology (EN): M.E. thritty, O.E. thritig, from thri, threo “three” + -tig “group of ten, -ty.”

Etymology (PE): Si, from Mid.Pers. sih; O.Pers. *ciθas nominative singular of *ciθant-; Av. θrisant- “thirty;” cf. Skt. trimśát- “thirty.”