[for.ty] n, pl forties [ME fourty, adj., fr. OE feowertig, fr. feowertig group of 40, fr. feower four + -tig group of 10; akin to OE tien ten] (bef. 12c) 1: four times ten
2. pl: the numbers 40 to 49; specif: the years 40 to 49 in a lifetime or century
3: the third point scored by a side in a game of tennis -- for.ti.eth adj or n -- forty adj -- forty pron, pl in constr -- for.ty.ish adj