"one who substitutes for another," 1928, from the verbal phrase, attested from 1904 in show business slang in the sense "to substitute, to fill the place of another," from stand (v.) + in (adv.).
Entries linking to stand-in
stand v.
Old English standan "occupy a place; stand firm; congeal; stay, continue, abide; be valid, be, exist, take place; oppose, resist attack; stand up, be on one's feet; consist, amount to" (class VI strong verb; past tense stod, past participle standen), from Proto-Germanic *standanan (source also of Old Norse standa, Old Saxon and Gothic standan, Old High German stantan, parallel with simpler forms, such as Swedish stå, Dutch staan, German stehen [see discussion in OED]), from *stathula, from PIE root *sta- "to stand, make or be firm."
Sense of "to exist, be present" is attested from c. 1300. Meaning "encounter without flinching" is from 1590s; weaker sense of "put up with" is from 1620s. Meaning "to submit" (to chances, etc.) is from c. 1700. Meaning "to pay for as a treat" is from 1821. Meaning "become a candidate for office" is from 1550s. Nautical sense of "hold a course at sea" is from 1620s. Meaning "to be so high when standing" is from 1831.
Stand back "keep (one's) distance" is from c. 1400. Phrase stand pat is from poker (1882), earlier simply stand (1824 in other card games). To stand down is from 1680s, originally of witnesses in court; in the military sense of "come off duty" it is first recorded 1916. To let (something) stand is from c. 1200. To stand for is c. 1300 as "count for;" early 14c. as "be considered in lieu of;" late 14c. as "represent by way of sign;" sense of "tolerate" first recorded 1620s. Phrase stands to reason (1620) is from earlier stands (is constant) with reason.
in adv., prep.
a Middle English merger of Old English in (prep.) "in, into, upon, on, at, among; about, during;" and Old English inne (adv.) "within, inside," from Proto-Germanic *in (source also of Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE root *en "in." The simpler form took on both senses in Middle English.
Sense distinction between in and on is from later Middle English, and nuances in use of in and at still distinguish British and American English (in school/at school). Sometimes in Middle English shortened to i.
The noun sense of "influence, access (to power or authorities)," as in have an in with, is first recorded 1929 in American English. to be in for it "certain to meet with something unpleasant" is from 1690s. To be in with "on friendly terms with" is from 1670s. Ins and outs "intricacies, complications of an action or course" is from 1660s. In-and-out (n.) "copulation" is attested from 1610s.