Green’s Dictionary of Slang

1. (also soup) to accelerate.

1940		R.L. Bellem ‘Death’s Passport’ in Goodstone Pulps (1970) 115/1: I climbed into my bucket and souped the kidneys out of it.
1943		R.L. Bellem ‘Feature Snatch!’ Dan Turner - Hollywood Detective Feb. 🌐 We piled into my heap and I souped the cylinders.

2. (orig. of engines) to increase in power, to aggrandize in some manner.

1949–51		in M. Daly Profile of Youth 49: Sure I’m going to soup it up. I may add a little chrome here and there on the outside, but not very much. With a hot rod, it’s what is under the hood that counts.
1959		E. De Roo Go, Man, Go! 9: You had to tear it down, soup it up and strip it for speed.
1969		P. Boyle All Looks Yellow to the Jaundiced Eye 103: Don’t soup it up too much.
1978		R. De Christoforo Grease 70: Just wait ’till I give it a paint job and soup up the engine.
1983		J. Byrell (con. 1959) Up the Cross 76: He used to drive hotrods, which he’d souped up himelf.
1994		G. Indiana Rent Boy 16: What about souping up that lethal injection thing in Florida with some magic mushrooms.
2012		T. Piccirilli Last Kind Words 27: [He] was known for souping up stolen cars for strings putting together bank heists. He’d fine-tune engines until they sang [ibid.] 110: [H]e was still souping cars for heisters.
2020		S.A. Crosby Blacktop Wasteland 233: Kelvin and him as kids souping up their bicycles.

3. (US) to stimulate a woman’s vagina.

1959		E. De Roo Go, Man, Go! 37: ‘You savvy drag talk? [...] Then give a little listen. Bend over my mill.’ She laughed contentedly. ‘Soup me up and test my torque.’.
1995		(con. 1985–90) P. Bourjois In Search of Respect 210: Luis busted out a lot of virgins. He used to soup up the girls, and bone them instantly.

4. (US black) to encourage, to persuade.

2008

in J. Miller Getting Played 100: ‘There’s a crowd around, like they’ll try to soup the girl or boy up to hit them...try to see the action going on’.