Green’s Dictionary of Slang

caf up v.

[SE caffeine]

(US campus) to ingest caffeine, usu. in the form of coffee, to promote one’s energy.

[US]Eble Campus Sl. Spring 2: caffed up – means hyped-up with caffeine from coffee.
[US]Eble Sl. and Sociability 64: caf up, ‘drink coffee or eat something containing caffeine for energy’.