"to make the characteristic cry of a cow, to low," 1540s, of imitative origin (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.
Montreal
monument
monumental
monumentalize
-mony
moo
mooch
moocher
mood
moodiness
moody