Green’s Dictionary of Slang

a woman or homosexual man who is simultaneously fellating one man while having intercourse, from the rear, with a second; thus roasting/spit-roasting n., this practice.

[1986		Stars mag. Feb. cover: [headline] Chicken on a Spit].
1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: spit roast n. To be simultaneously skewered at both ends by pork swords (qv); a sausage jockey (qv) who plays a pink oboe (qv) whilst still in the saddle.
2003		Guardian G2 7 Oct. 5/1: According to several newspapers, ‘roasting’ is supposed to suggest the stuffing of a piece of meat. More likely [...] is that it refers to spit roasting, with an insertion at either end [...] the woman here is, essentially, a tube to permit contact between the men.
2007		E. Morrison Swung 303: Spit-roasting. Anal penetration. Double penetration. How had he even let her talk him into this?
2012		R. Milward Kimberly’s Capital Punishment (2023) 63: [M]e treating the two-headed monster to a sympathetic, gin-soaked spit-roasting session.

of a woman, having vaginal or anal intercourse with one man and fellating another; of a man, being sodomized and simultaneously fellating another man.

1972		B. Rodgers Queens’ Vernacular 128: The one in the middle of a threesome [...] chicken on a spit.
2000		M. Manning Get Your Cock Out 93: Sonny Darklord was on the spit. Gerry Enchilada had bagsied the jacksie and was thrusting hard up the kid’s chufter chute. Robbierre Le Vomiteur was fucking pretty boy’s face.

to have sexual intercourse.

1720		in D’Urfey Pills to Purge Melancholy VI 92: Sometimes I am a Cook, / And in Fleet-Street I do dwell Sir / [...] /And if a dainty Lass comes by, / And wants a dainty bit Sir; / I take four Quarters in my Arms, / And put them on my Spit Sir.
1836		‘Johnny’s Rolling Pin’ in Gentleman’s Private Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) III 377: And to Johnny the footman she used to think fit, / To go every morn to be out on the spit.