surrogate (n.)

early 15c., "a substitute, person appointed or deputed by authority to act for another," from Latin surrogatus, past participle of surrogare/subrogare "put in another's place, substitute." This is from an assimilated form of sub "in the place of, under" (see sub-) + rogare "to ask, propose," which is apparently a figurative use of a PIE verb meaning literally "to stretch out (the hand)," from root *reg- "move in a straight line."

Originally usually of bishops or judges; general sense of "person or thing that acts for or takes the place of another" is by 1640s. The meaning "woman pregnant with the fertilized egg of another woman" is attested from 1978 (from 1972 of animals; surrogate mother in a psychological sense is from 1971). As an adjective from 1630s.