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English definition of “fill in”

fill in

phrasal verb with fill     /fɪl/ verb
to do someone else's work for them because they cannot or will not do it themselves: Volunteers would fill in for teachers in the event of a strike. I'm not her regular secretary - I'm just filling in.Replacing and exchanging
(Definition of fill in from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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