Is it ensure, insure, or assure?
c.1400, originally Scottish, from Middle Dutch kronen "to lament, mourn," perhaps imitative. Originally "to bellow like a bull" as well as "to utter a low, murmuring sound" (mid-15c.). Popularized by Robert Burns. Sense evolved to "lament," then to "sing softly and sadly." Related: Crooned; crooning.
[1460+; fr Scots dialect; related to Dutch kreunen, ''groan, whimper'']