Entry 2 senses · 2 variants Webster, 1913 mellowing · IPA /mˈɛloʊɪŋ/ adj. n. 01 adj. pr. p. of mellow. 1. pr. p. of mellow. See: mellow 02 n. The act or process of acquiring desirable qualities by being left undisturbed for some time. 1. The act or process of acquiring desirable qualities by being left undisturbed for some time. Syn. ripening, aging, ageing.