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