Douglas Harper's Etymology Dictionary
caco-
before vowels cac-, word-forming element meaning "bad, ill, poor" (as in cacography, the opposite of calligraphy), from Latinized form of Greek kako- a hard-working prefix in ancient Greek, from kakos "bad, evil," considered by etymologists probably to be connected with PIE *kakka- "to defecate" (see caca).
Wiktionary
caco-
pre. bad, defective