Lewis & Short

ef-frēnus, a, um (also effrenis mula, Plin. 8, 44, 69, § 171; Sen. de Ira, 3, 15, 2), adj. [ex-frenum], free from the bridle, unbridled (not ante Aug., and mostly poet. for effrenatus; cf. also: infrenus, infrenatus).

  1. I. Lit.: equus, Liv. 4, 33, 7.
  2. II. Trop., unbridled, unrestrained: gens, Verg. G. 3, 382: amor, Ov. M. 6, 465: juventa, Stat. Achill. 1, 277: profatu, id. Silv. 5, 3, 103.