Webster’s Revised Unabridged Dictionary · 1913
Moo
Moo (mō), a., adv., & n. See Mo. [Obs.] Chaucer.
Moo (mo͞o), v. i. [imp. & p. p. Mooed; p. pr. & vb. n. Mooing.] [Of imitative origin.] To make the noise of a cow; to low; — a child's word.
Moo, n. The lowing of a cow.