fill in

Verb

fill in (third-person singular simple present fills in, present participle filling in, simple past and past participle filled in)

1. (idiomatic, transitive) To fill; to replace material that is absent or has been removed.
   • After you're done laying the pipe, fill in the trench.
   • The decking of the bridge consisted of steel troughing over an average clear span of 26 ft., the troughing being filled in with concrete to form the road surface.1958 May, “Widening at Dun Laoghaire, C.I.E.”, in Railway Magazine, page 342
2. (idiomatic, transitive) To inform somebody, especially to supply someone missing or missed information.
   • If you know anything about this, maybe you can fill me in.
   • Wait, uh - I-I-I should go distract Caine so he doesn't l-listen in on this. Uh, j-just fill me in afterwards.2026 March 20, Gooseworx, “hjsakldfhl” (20:32 from the start), in The Amazing Digital Circus, episode 8, spoken by Jax (Michael Kovach)
3. (idiomatic, intransitive) To substitute for somebody or something.
   • He can't go on vacation very often because there is nobody to fill in for him.
   • By 6 am, Jodi hadn’t arrived yet, and Kuns filled in for her after a second call went unanswered.2026 August 10, Nicholas Muhoro, “Jodi Huisentruit, 27, Vanished From Her Apartment Parking Lot in 1995. Police Found Signs of a Struggle”, in Skip Boring, archived from the original on 11 Aug 2026
4. (UK, idiomatic, transitive) To complete a form or questionnaire with requested information.
   • I've got a form that is to be filled in in ink.
   • The findings emerged from questionnaires filled in by 2,211 staff in 145 wards of 55 hospitals in England and Wales and 105 observations of care of dementia patients. Two-thirds of staff said they had not had enough training to provide proper care, 50% said they had not been trained how to...2011 December 16, Denis Campbell, “Hospital staff 'lack skills to cope with dementia patients'”, in Guardian

   Synonyms:

   fill out

5. (idiomatic, slang) To beat up; to physically assault.
   • Talk to me like that again and I'll fill you in!
   • It's just a rumour that was spread around town
     Somebody said that someone got filled in
     For saying that people get killed in
     The result of this shipbuilding1982, Elvis Costello, Shipbuilding