Adjective. Tending to produce or aggravate acne. From the Latin roots comedo (blackhead) and -genic. Comedo means glutton in Latin, and came to mean blackhead from a comparison of the worm-like shape of the waxy material that can be squeezed from a blackhead to a worm believed to feed on the body. Nice.
In U.S. (FDA-regulated) product labelling, the more commonly encountered "non-comedogenic" means "does not clog pores or cause blemishes."
dictionary.com and Galderma helped out on this one.