1350-1400;Middle English < Middle Frenchterçuel < Vulgar Latin*tertiolus, equivalent to Latinterti(us) third + -olus-ole1; probably so called because the male is about one third smaller than the female
"male falcon," c.1381, from O.Fr. tercel (c.1200), from M.L. tertiolus, from L. tertius "third." Various theories as to why it is called this; one says it's because the males are a third smaller than the females, another because a third egg in the nest (smaller than the other two) is believed always to produce a male bird.