Green’s Dictionary of Slang

bananas adj.

[? one’s mind or sexuality is ‘bent out of shape’]

1. [1930s+] homosexual.

2. [1950s+] of a person, crazy, eccentric.

3. [1990s+] of an object, crazy, unbelievable.

In phrases

drive bananas (v.)

[1970s+] (orig. US) to drive mad, to infuriate.

flip one’s banana (v.) (also flip one’s raspberry) [SE flip; note flip v.4 (2)]

[1970s] (US) to go suddenly insane, to lose control of one’s emotions.

go bananas (v.) [1960s+] (orig. US)

1. to lose emotional control [image of an over-ripe banana that ‘goes soft’].

2. to delight in something absolutely; usu. as go bananas over.