Lewis & Short

in-dulco, āvi, ātum, 1, v. a., to make sweet, to sweeten (late Lat.).

  1. I. Lit.: aquam amaram, Tert. adv. Jud. 13; Vulg. Sirach, 38, 5.
  2. II. Trop., to speak sweetly, Vulg. Sirach, 12, 15.