Yours, Etc.: Origins and Uses of 8 Sign-Offs
before vowels cac-, word-forming element meaning "bad, ill, poor" (e.g. 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).
caco- or cac-