1530, “to recount, tell,” from L. relatus, used as pp. of referre (see refer), from re- “back, again” + latus (see oblate (n.)). Meaning “to establish a relation between” is from 1771. Sense of “to feel connected or sympathetic to” is attested from 1950, originally in psychology jargon. Related in the sense of “connected by blood or marriage” is from 1702.
1560s, from L. reciprocus “returning the same way, alternating,” from pre-L. *reco-proco-, from *recus (from re- “back” + -cus, adjective formation) + *procus (from pro- “forward” + -cus, adjective formation).