Entry 3 senses · 3 variants Webster, 1913 Moo /(mō)/ · IPA /muː/ a., adv., & n. v. i. n. 01 a., adv., & n. See Mo. 1. See Mo.[Obs.] See: Mo 02 v. i. To make the noise of a cow; to low; -- a child's word. imp. & p. p. Mooed; p. pr. & vb. n. Mooing 1. To make the noise of a cow; to low; -- a child's word. 03 n. The lowing of a cow. 1. The lowing of a cow.