Green’s Dictionary of Slang

1. in hell.

1840		R. Barham ‘The Brothers of Birchington’ Ingoldsby Legends (1847) 259: They are bruited about, doubtless, much more ‘down stairs,’ / Where Old Nick has a register-office, they say.
1955		E. Hoagland Cat Man 302: ‘You think you’re better. Dont’cha? You alluz think you’re better. But you’re not. You may be goin’ downstairs too. You’re no better’.

2. (also below stairs) in the urino-genital area.

1702		J. Floyer Essay to Prove Cold Bathing 283: One may look brisk, with Cherry-Cheek, And yet below Stairs very weak. That Woman’s in a doubtful Case That builds her hopes upon a Face; As one was cheated, when she chose A Husband by the length of’s Nose.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1963		in T.I. Rubin Sweet Daddy 132: No taking off no clothes, no touching downstairs.
1974		J. Lahr Hot to Trot 16: Rules of the Game – French kiss [...] Downstairs [...] Hand job.
2001		J. Stahl Plainclothes Naked (2002) 4: At least her grandaddy, whatever his other faults, had not spent a lot of time boring her with what was happening downstairs.
2013		NSFWCorp 18 Sept. 🌐 A man obsessed with putting condoms on each and every porn cock destined for a downstairs orifice .
2015		J. Stahl OG Dad 72: Mommy’s going to be completely numb downstairs.
2015		I. Welsh Decent Ride 114: Wir up in this smart room oan the bed. Ah’m right doonstairs, lickin away like a Jambo at plate gless.

3. in the guts or belly.

1913–14		Van Loan ‘No Business’ Taking the Count 167: A glove thudded [...] into the unarmored region known as the pit of the stomach. ‘Thatta boy! howled the Cricket. ‘Downstairs!’.
1937		J. Curtis There Ain’t No Justice 23: Hogan caught a nice one downstairs in the kitchen and had stepped back.
1938		D. Runyon ‘The Big Umbrella’ Runyon on Broadway (1954) 563: ‘Downstairs, Jonas,’ I say. ‘In the elly-bay, Jonas.’.
2000		Guardian 16 Feb. 18: You all know what happens downstairs when that happens.