"Banana head" means a stupid person according to Farlex Idioms. Where did this idiom come from?
n. a stupid person. (Usually objectionable.) Ask that banana-head why she is wearing a coat like that in July.
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From J.E. Lighter, Random House Historical Dictionary of American Slang (1994):
bananahead n. a fool; idiot. [First two cited occurrences:] 1949 W.R. Burnett Asphalt Jungle 205: You...big...bananahead! 1963 E.M. Miller Exile to Stars 220: But I'll be damned if I'll have that banana-head Blair Winsted in here.
The epithet appears twice in a short span of text in The Asphalt Jungle (1949) [combined snippets]:
"Haven't you bothered me enough?" cried Angela. "You big ... bananahead!"
As she groped for an epithet, Andrews had winced inwardly, expecting the worst and not wanting to hear it. He had romantic feelings about pretty young women, and these feelings were always being outraged. He felt such marked relief that he burst out laughing.
Angela slammed the door in his face, but opened it again in in a moment, her anger considerably diminished. After all, this young cop was quite a spectacle. Big, roughly handsome, and with shoulders like a football-player. Besides ... who knows? She might need him. But to give in too fast was never very wise; so she narrowed her eyes again.
"Go ahead, laugh!" she cried. "But bananahead it is! ..."
In this exchange, it appears that Angela has made up the insult on the spot, and that Andrews has never heard it before.
An interesting alternative source of the term appears in the context of a series of preventive folk measures taken by members of an unidentified culture, in Folk, volumes 8–9 (1966) [snippet view]:
To plant bananas (lest the child have an extended head at birth; this is also popularly known as a "banana head" in English).
Unfortunately, the lack of context for this excerpt makes it difficult to assess what influence, if any, the phenomenon of neonatal "banana head" may have had on the emergence of the insult term.
banana-head n. a stupid person. (Usually objectionable.) Kelly can be such a banana-head! Ask that banana-head why she is wearing a coat like that in July.
According to World Wide Words, the origin of banana head is unknown
Among others, banana has been used as an obvious slang term for the penis (and also for a dollar, for less obvious reasons); it was once a name for a slapstick comedian in vaudeville, leading to those evocative terms top banana for the starring act and second banana for a supporting performer or straight man. There’s banana oil for nonsense, baloney or hypocritical talk, a close relative of apple-sauce; a banana ball is one that curves in the air; a bananahead is a fool. All these are American, you will note.
So information is available but I failed to identify the idiom's origin.