CHECK IN

{v.} 1a. To sign your name (as at a hotel or convention). * /The last guests to reach the hotel checked in at 12 o'clock./ Contrast: CHECK OUT. 1b. {informal} To arrive. * /The friends we had invited did not check in until Saturday./ 2. To receive (something) back and make a record of it. * /The coach checked in the football uniforms at the end of the school year./ * /The students put their books on the library desk, and the librarian checked them in./