disappointyoudaoicibaDictYouDict[disappoint 词源字典]
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 etymology, disappoint origin, 英语词源]
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.