"to make the characteristic cry of a cow, to low," 1540s, of imitative origin (compare Latin mūgire "to low, moo," Lithuanian mūkiu "to bellow," Middle High German mūhen "to low, bellow," and see PIE root *gwou-). Related: Mooed; mooing. The noun "the low of a cow" is from 1789. Baby-talk moo-cow (n.) "a cow" is attested from 1812.
