Green’s Dictionary of Slang

crackers adj.

[cracked adj. (4)]

mad, crazy; thus crackers about, obsessed with; get the crackers, to go mad.

[UK](con. WWI) Fraser & Gibbons Soldier and Sailor Words 65: Crackers, To Get The: To go off one’s head. Mad.
[UK]Operator 1384 Scourge of the Desert 122: ‘Crackers!’ whispered McCann. ‘You can betcha sweet life that guy’s gone cafard’ .
[UK]G. Kersh They Die with Their Boots Clean 177: Nuts, I tell you, nuts, crazy, crackers!
[Aus]Cusack & James Come in Spinner (1960) 299: You’re crackers, absolutely crackers. Fancy missing a chance to go and live in the States.
[Aus]T.A.G. Hungerford Riverslake 182: She’s crackers about the feller that got knocked down.
[UK]K. Waterhouse There is a Happy Land (1964) 16: He’s not simple, he’s blinking crackers. [Ibid.] 139: Marion was dead crackers about the pictures.
[UK]T. Parker Frying-Pan 88: Sometimes I reckon I must definitely be crackers.
[Aus]M. Bail Homesickness (1999) 204: Everyone’s either crackers or they’re getting at you. I’m not stupid.
[UK]K. Lette Foetal Attraction (1994) 16: You’re crackers! We can’t go in there.
[UK]M. Coles More Bible in Cockney 133: You’re a blinkin’ loony, Paul! You’ve studied so bloomin’ much that it’s made you completely crackers!

In phrases

go crackers (v.)

to go mad, to become insane, eccentric.

[UK]Sunderland Dly Echo 29 Apr. 1/7: Man: ‘I think my wife has gone crackers!’ Chairman: ‘Crackers?’ Man: ‘Yes, “batchy,” bats in the belfry’.
[UK]J. Curtis Gilt Kid 238: Don’t go crackers.
[UK]A. Sillitoe ‘On Saturday Afternoon’ Loneliness of the Long-Distance Runner (1960) 102: I thought he’d gone crackers in a quiet sort of way.
[UK]A. Sillitoe Start in Life (1979) 196: People may think I’ve gone crackers.
in Williams & Jenkins Rekindled 137: I’d go crackers if I had to be away for more than two days.
[UK]G. Burn Happy Like Murderers 202: Rose used to go crackers at me to get rid of them.
K Badman Beatles: The Dream Is Over 141: If you’re stuck in New York, you have to somehow look within yourself, otherwise you’d go crackers.