c. 1400 (late 13c. as a surname), from Anglo-French spinache, Old French espinache (14c., Modern French épinard, from a form with a different suffix), from Old Provençal espinarc, which perhaps is via Catalan espinac, from Andalusian Arabic isbinakh, from Arabic isbanakh, from Persian aspanakh "spinach." But OED is not convinced the Middle Eastern words are native, and based on the plethora of Romanic forms pronounces the origin "doubtful."
Popeye, the spinach-eating superman, debuted in 1929. Old folk etymology connected the word with Latin spina (see spine) or with Medieval Latin Hispanicum olus. For pronunciation, see cabbage. In 1930s colloquial American English, it had a sense of "nonsense, rubbish," based on a famous New Yorker cartoon of Dec. 8, 1928. Related: spinaceous.
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 capocchia, diminutive of capo. The cabbage-butterfly (1816) is so called because its caterpillars feed on cabbages and other cruciferous plants.