What does Boxing Day have to do with boxing?
early 15c., from Latin surrogatus, past participle of surrogare "put in another's place, substitute," from sub "in the place of, under" + rogare "to ask, propose" (see rogation). 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).
surrogate sur·ro·gate (sûr'ə-gĭt, -gāt', sŭr'-)
One that takes the place of another; a substitute.
A person or an animal that functions as a substitute for another, as in a social or family role.
A figure of authority who takes the place of the father or mother in a person's unconscious or emotional life.
A surrogate mother.