Green’s Dictionary of Slang

1. (US Und.) to get acquainted.

1886

‘Lady Kate, the Dashing Female Detective’ in Roberts et al. Old Sleuth’s Freaky Female Detectives (1990) 14/2: Kate made up her mind to ‘lay in’ with Randolph Cummings, and take his full measure. She made several attempts to get acquainted with him, but got the ‘dead shake’ every time.

2. (US) to act as confederates.

1908

‘Old Sleuth’ Dock Rats of N.Y. (2006) 103: ‘Spencer Vance and you were “laying in” together?’ ‘Well, yes.’.

3. (US) to obtain, to ‘look out for’.

1908

‘Old Sleuth’ Dock Rats of N.Y. (2006) 104: ‘I’ve got everything dead.’ ‘I see you have.’ ‘Then it’s for you to lay in for all the favors you can get.’.

4. (US) to stay, esp. in the context of hiding from pursuit.

1929

W.R. Burnett Little Caesar (1932) 238: I’m looking for a place to lay in.

5. (US prison) to stay in one’s cell when one might usu. be out of it; also as n.

1933		Ersine Und. and Prison Sl.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 209: lay in, v. – to stay in the cell for official reasons.
1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
1999		E. Bunker Mr Blue 388: [He] had a medical lay-in. He was taking a nap with his cell gate open.