mid-15c., caboge, from Middle French caboche "head" (in dialect, "cabbage"), from Old French caboce "head," a diminutive from Latin caput "head" (see capitulum). Introduced to Canada 1541 by Jacques Cartier on his third voyage. First written record of it in U.S. is 1660s.
The decline of "ch" to "j" in the unaccented final syllable parallels the common pronunciation of spinach, sandwich, Greenwich, etc. The comparison of a head of cabbage to the head of a person (usually disparaging to the latter) is at least as old as Old French cabus "(head of) cabbage; nitwit, blockhead," from Italian cappuccio, diminutive of capo.