Is it ensure, insure, or assure?
c.1200, from Old French chartre (12c.) "charter, letter, document, covenant," from Latin chartula, literally "little paper," diminutive of charta, carta "paper, document" (see chart (n.)).
early 15c., "provide with a charter," from charter (n.). Meaning "to hire" is attested from 1806. Related: Chartered; chartering.