Etymology
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fill-in (n.)

"substitute," 1918 (as an adjective, 1916), from verbal phrase; see fill (v.), in (adv.). Earlier as a noun was fill-up (1811).

updated on October 10, 2017

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Definitions of fill-in from WordNet

fill-in (n.)
someone who takes the place of another (as when things get dangerous or difficult);
we need extra employees for summer fill-ins
Synonyms: stand-in / substitute / relief / reliever / backup / backup man
From wordnet.princeton.edu, not affiliated with etymonline.