Is it ensure, insure, or assure?
early 13c., from Old French glu "birdlime" (12c.), from Late Latin glutem (nominative glus) "glue," from Latin gluten "glue, beeswax," from PIE *gleit- "to glue, paste" (cf. Lithuanian glitus "sticky," glitas "mucus;" Old English cliða "plaster"), from root *glei- "to stick together" (see clay). In reference to glue from boiled animal hoofs and hides, c.1400. Glue-sniffing attested from 1963.
late 14c., from Old French gluer, from glu (see glue (n.)). Related: Glued; gluing.