mid-14c., "what one deserves, just desserts," from Anglo-French and Old North French reward, rouwart, back-formation from rewarder (see reward (v.)).
The meaning "return or payment for service, hardship, etc.," also "something given in recognition of merit, virtue, etc., a prize" is from late 14c. Also from late 14c. sometimes "punishment, recompense for evil-doing." The sense of "sum of money in exchange for capture of a criminal or fugitive or for return of a lost item" is from 1590s.
A doublet of regard (n.), reward also was used in Middle English in the senses now given to that word: "a regarding, heeding, notice, observation," also "respect, esteem."
c. 1300, rewarden, "to grant, bestow;" early 14c. "to give as prize or compensation," from Anglo-French and Old North French rewarder "to regard, reward" (Old French regarder) "take notice of, regard, watch over." This is from re-, here perhaps an intensive prefix (see re-), + warder "look, heed, watch," from Frankish or some other Germanic language, from Proto-Germanic *wardon "to guard" (from PIE root *wer- (3) "perceive, watch out for").
Originally any form of requital, good or bad, for service or evil-doing. A doublet of regard (v.), reward was used 14c.-15c. in the senses now given to that word: "look at; care about; consider." Related: Rewarded; rewarding.