Green’s Dictionary of Slang

roll in the hay n.

also roll-around, roll in the meadow, toss in the hay, tumble in the hay
[SE roll/tumble n. (2) + SE hay]
[1940s+] (orig. US)

1. sexual intercourse, with the implication of spontaneity, adultery or the open air.

2. a person viewed as a possible sexual partner.