Green’s Dictionary of Slang

potion n.

(W.I.) a large quantity.

1995

Francis-Jackson Official Dancehall Dict. 41: Potion amount, numbers etc., of large quantities: u. a potion a girls.

SE in slang uses

In phrases

have a potion (v.)

to have sexual intercourse.

2000

J.J. Connolly Layer Cake 260: Johnny’s flirtin with her [...] and she’s gettin all worked up cos she probably hasn’t had a potion for stretches.