"moving backward and forward, alternating; mutually exchanged or exchangeable; having an interchangeable character or relation, mutually equivalent and correspondent," 1560s, with -al (1) + stem of Latin reciprocus "returning the same way, alternating," from pre-Latin *reco-proco-, from *recus (from re- "back;" see re-, + -cus, adjective formation) + *procus (from pro- "forward," see pro-, + -cus). Related: Reciprocally. The noun meaning "that which is reciprocal" (to another) is from 1560s.
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