Is it ensure, insure, or assure?
1650s, from or along with the noun (1630s), first used as a cribbage term; as a verb, "to spoil (someone's) score." Origin obscure, it was believed in 17c. to be "a word signifying nothing;" perhaps it s a thinned form of balk "to hinder." Meaning "to defraud" is first recorded 1670s. Related: Bilked; bilking.