Green’s Dictionary of Slang

1. sexual intercourse (usu. adulterous or with a prostitute) in the afternoon.

[1890		Bird o’ Freedom Jan. 29 n.p.: His mattie is a pretty girl - as fair as fair can be / And every time he comes to call he has a Mat-on-knee].
1944		in P. Smith Letter from My Father (1978) 429: When the husband was on the road we would have matinees – she was a swell lay.
1972		B. Rodgers Queens’ Vernacular 91: brief sex act accomplished at noontime [...] matinee [trick]. [Ibid.] 132: matinee sexual encounter taking place during the early afternoon.
2002		D.H. Sterry Chicken (2003) 111: Who the hell orders a chicken at seven o’clock on Monday. It’s always midnights or matinees.

2. a prostitute’s client who makes his appointments for the afternoon.

1958

Murtagh & Harris Cast the First Stone 12: Some commuting businessmen, called matinées, reject the night hours altogether, and come afternoons between two and four-thirty.