Lewis & Short

† mu, interj., = μῦ, a slight sound made with the closed lips (ante-class.): neque, ut aiunt, mu facere audent,

  1. I. to mutter, make a muttering, Enn. ap. Varr. L. L. 7, § 101 Müll. (Enn. p. 175, n. 10 Vahl.): nec mu facere audet, Lucil. ap. Charis. p. 213 P.: nec dico nec facio mu, Enn. ap. Don. ad Ter. And. 3, 2, 25 (Enn. p. 175, n. 8 Vahl.; but not in Plaut. Stich. 1, 3, 102, v. Ritschl ad h. l.).
  2. * II. A sound of lamentation: mul perii hercle: Afer est, Plaut. Fragm. ap. Charis. p. 213 P.