disappointyoudaoicibaDictYouDict
disappoint: [15] Disappoint (a borrowing from French désappointer) originally meant ‘remove from a post or office, sack’ – that is, literally, ‘deprive of an appointment’; ‘A monarch … hath power … to appoint or to disappoint the greatest officers’, Thomas Bowes, De La Primaudraye’s French academie 1586. This semantic line has now died out, but parallel with it was a sense ‘fail to keep an appointment’, which appears to be the ancestor of modern English ‘fail to satisfy, frustrate, thwart’.
disappoint (v.)youdaoicibaDictYouDict
early 15c., "dispossess of appointed office," from Middle French desappointer (14c.) "undo the appointment, remove from office," from des- (see dis-) + appointer "appoint" (see appoint).

Modern sense of "to frustrate expectations" (late 15c.) is from secondary meaning of "fail to keep an appointment." Related: Disappointed; disappointing.