[thir.ty] n, pl thirties [ME thritty, fr. thritty, adj., fr. OE thritig, fr. thritig group of 30, fr. thrie three + -tig group of ten; akin to OE tien ten] (bef. 12c) 1: three times ten
2. pl: the numbers 30 to 39; specif: the years 30 to 39 in a lifetime or century
3: a sign of completion: end--usu. written 30 "wrote ~ on the last page of the story"
4: the second point scored by a side in a game of tennis -- thir.ti.eth adj or n -- thirty adj -- thirty pron, pl in constr -- thir.ty.ish adj