BT San 42a(2) :הא למהוי כי יתרא, הא למהוי כי נפיא‏ : this one is to be like (a bow with a) bowstring (i.e., half-moon) that one to be like a sieve (i.e. round)
See the entire entry for yṯar (īṯar), yatrā n.m. rope; bow-string