croon (v.)

c. 1400, "to speak or sing softly," originally Scottish; compare Middle Dutch kronen "to lament, mourn," Old High German kronen "babble." The relationship among them is obscure, perhaps all are imitative. In early use also "to bellow like a bull" as well as "to utter a low, murmuring sound" (mid-15c.). Popularized by Robert Burns. The medieval sense evolution might be from "to lament" to "sing softly and sadly." Related: Crooned; crooning.