Is it ensure, insure, or assure?
1825, the first element possibly from a diminutive of cub "stall, pen, cattle shed, coop, hutch" (1540s), a dialect word with apparent cognates in Low German (e.g. East Frisian kubbing, Dutch kub). Or related to cuddy "small room, cupboard" (1793), originally "small cabin in a boat" (1650s), from Dutch kajuit, from French cahute. Or perhaps simply a children's made-up word.