'fill in' definitions:

Definition of 'fill in'

(from WordNet)
verb
Supply with information on a specific topic; "He filled me in on the latest developments"
verb
Represent the effect of shade or shadow on [syn: shade, fill in]
verb
Be a substitute; "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream--we are on a strict diet" [syn: substitute, sub, stand in, fill in]
verb
Write all the required information onto a form; "fill out this questionnaire, please!"; "make out a form" [syn: complete, fill out, fill in, make out]

Synonyms of 'fill in'

From: Moby Thesaurus