1225-75; (noun) Middle English < Medieval Latin,Latin: it concerns, literally, it is between; replacing interesse < Medieval Latin,Latin: to concern, literally, to be between; (v.) earlier interess as v. use of the noun; see inter-, esse
early 15c., earlier interesse (late 14c.), from Anglo-Fr. interesse "what one has a legal concern in," from M.L. interesse "compensation for loss," from L. interresse "to concern, make a difference, be of importance," lit. "to be between," from inter- "between" + esse "to be." Form influenced 15c. by O.Fr. interest "damage," from L. interest "it is of importance, it makes a difference," third pers. sing. present of interresse. Financial sense of "money paid for the use of money lent" (1529) earlier was distinguished from usury (illegal under Church law) by being in ref. to "compensation due from a defaulting debtor." Meaning "curiosity" is first attested 1771.