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Related to fill in: fill up
1. To provide information or details, usually by actually writing something in a blank space on a test, form, etc. In this usage, a noun or pronoun can be used between "fill" and "in." Fill in the blank with what you think is the correct answer. Ugh, I had to fill in so much paperwork on my first day of work.
2. To substitute for someone or something. In this usage, "fill in" is followed by "for." Can you fill in for me on the conference call? I have to leave early today for a doctor's appointment.
3. To add to something so that it looks more uniform in appearance. In this usage, a noun or pronoun can be used between "fill" and "in." Your eyebrows are looking a little sparse—can I fill them in? The landscaper is coming today to fill in all these holes in the lawn.
4. To inform one of something. In this usage, a noun or pronoun can be used between "fill" and "in." Fill me in—what happened at the party last night?
fill someone in (on someone or something)
to tell someone the details about someone or something. Please fill me in on what happened last night. Please fill in the committee on the details.
fill something in
1. to add material to an indentation, hole, etc., to make it full. You had better fill the crack in with something before you paint the wall. You should fill in the cracks first.
2. Fig. to write in the blank spaces on a paper; to write on a form. (See also Fill in the blanks) Please fill this form in. I will fill in the form for you.
fill in (for someone or something)
Fig. to substitute for someone or something; to take the place of someone or something. I will have to fill in for Wally until he gets back. I don't mind filling in.
[for an indentation, hole, etc.] to become full. The scar filled in after a few months. Will this hole in the ground fill in by itself, or should I put some dirt in?
1. Complete something, especially by supplying more information or detail. For example, Be sure to fill in your salary history. It is also put as fill in the blanks, as in We'll rely on Mary to fill in the blanks. Yet another related usage is fill someone in, as in I couldn't attend, so will you fill me in? The first term dates from the mid-1800s; the others from the first half of the 1900s. Also see fill out.
2. Also, fill in for. Take someone's place, substitute for. For example, The understudy had to fill in at the last minute, or I can't come but my wife will fill in for me. Also see fill someone's shoes.
1. To provide someone with essential or newly acquired information: I didn't receive the information in the mail—could you fill me in? Please fill in the new secretary about our rules.
2. To provide something, especially required information, in written form or on a document: The applicant filled the answers in on the registration form. The students filled in the test forms.
3. To cover completely the bounded surface of something: Fill in all the rectangles on the page with blue ink. The artist filled all the stencils in with pastels.
4. To act as a substitute; stand in: When I was sick, my colleague filled in. The understudy filled in for the sick actor last night.