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"something stored up," 1610s, from reserve (v.) or from French réserve, a Middle French back-formation from reserver. Meaning "self-imposed restraint on freedom of words or actions; habit of keeping back the feelings" is from 1650s.
reserve re·serve (rĭ-zûrv')
v. re·served, re·serv·ing, re·serves
To keep back, as for future use or for a special purpose.
To set or cause to be set apart for a particular person or use.
Held back, set aside, or saved.
Forming a reserve.