Green’s Dictionary of Slang

beaner n.2

[? Fr. bien]

(US) something excellent.

[US]L. Pound ‘A Second Word-List From Nebraska’ in DN III:vii 541: beaner, n. Term of appreciation, or compliment; equivalent to fine or excellent. ‘That new dress is a beaner.’.
[US]Wentworth & Flexner DAS.