Green’s Dictionary of Slang

banzai n.

a spree.

1909		Ade ‘Modern Japanee’ in Verses and Jingles (1911) 25: The Russian and the tough Chinee, / Can tell a lot regarding me, / Banzai! Hoopla! Japanee!
1914		‘Bartimeus’ ‘The ‘Look-See’’ in Naval Occasions 115: ‘What’s up with them all, sir?’ murmured the boy delightedly. ‘My Aunt! What a Banzai!’.
1929		F.C. Bowen Sea Sl. 7: Banzai Party, a party of naval men going ashore for a spree.
1932		Kipling Limits and Renewals 186: That’s how it was till the Squadron returned. [...] The banzai-parties came ashore, all hats and hosannas like a tax-payers’ treat.

In phrases

go banzai (v.)

to lose emotional control.

2000

N. Griffiths Grits 405: Memba he tried to chat Mairead up once an Colm wanted ta kill im? Went fuckin banzai ee did.