FILL IN

v. 1. To write words needed in blanks; put in; fill. You should fill in all the blanks on an application for a job. 2. informal To tell what you should know. The new boy didn't know the rules so Bob filled him in. The teacher filled in Mary about class work done while she was sick. 3. To take another's place; substitute. The teacher was sick and Miss Jones filled in for her.