bean, n.³ — Green’s Dictionary of Slang

bean n.³

[? corruption of ‘bee in one’s bonnet’]

(US) a foolish, silly notion.

c.1895		F. Norris Vandover and the Brute (1914) 41: ‘Ah, Dolly, you’ve got a bean,’ muttered Ellis.
1899		F. Norris McTeague (1958) 39: You got a bean about somethun, hey? Spit ut out.