Green’s Dictionary of Slang

1. (US, also cheque in) to die.

1909

W.M. Raine Brand Blotters (1912) 247: They couldn’t take me alive at all, and I reckon before I chequed in a few of them would.

2. (US, also check it in) to go to bed.

1969

Current Sl. IV:1 5: Check it in, v. To go to bed.

3. (US) to say hello, to greet.

1969		D. Di Prima Memoirs of a Beatnik 21: Frankie, an Italian racketeer of about thirty, checked in for a minute. ‘Honey [...] is dat guy bothering you?’ .
1970		E. Tidyman Shaft 124: Got to check in with these women.
1977		Eble Campus Sl. Apr. 1: check – Check in (Hello).
1997		Simon & Burns Corner (1998) 26: He checks in with Boo, who’s been working for him.

4. (UK/US Und.) to move from the general prison population into protective solitary confinement.

1992		Bentley & Corbett Prison Sl. 9: Checked-In also Check In When an inmate leaves the general population of the prison and enters the protective segregation unit, he is referred to as checked in.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Check In: To be placed into protective custody.