An Etymological Dictionary of Astronomy and Astrophysics

A cardinal number, 2 plus 1.

Etymology (EN): M.E.; O.E. threo, thrib, feminin and neuter of thri(e);

cf. O.Fris. thre, M.Du., Du. drie, O.H.G. dri, Ger. drei, Dan. tre), cognate with Pers. sé, as below.

Etymology (PE): Sé, from Mid.Pers. sé; Av. θrayô, θrayas, tisrô, θri; cf. Skt. tráya, tri, trini; Gk. treis, L. tres, Lith. trys, O.C.S. trye, Ir., Welsh tri, O.E. threo, as above; PIE base *trei-.