Green’s Dictionary of Slang

1. (also pepper-up) an amphetamine.

1937		Time 10 May 45/1: They call it ‘pepper-up’, ‘pep pills’.
1966		E. & S. Deak Grand Dictionnaire d’Americanismes (4th edn).

2. alcohol.

1938		in Mencken Amer. Lang. (1945) Supplement I 259: A cocktail was therefore what I suppose today would be called a pepper-upper.
1942		Berrey & Van den Bark Amer. Thes. Sl.
1957		E. & S. Deak Grand Dictionnaire d’Americanismes (2nd edn).

3. a cheerleader.

1959

E. De Roo Young Wolves 20: Then I can practise cheers with the Pepper-Uppers?