Is it ensure, insure, or assure?
1510s, "to lift up" (the voice), probably from late 14c. West Midlands dialect lulten "to sound an alarm," of unknown origin. Possible relatives include Norwegian lilla "to sing" and Low German lul "pipe." It is possible that the whole loose group is imitative. Sense of "sing in a light manner" is first recorded 1786. Related: Lilted; lilting. As a noun, 1728, "lilting song," from the verb. As "rhythmical cadence," 1840.