= I should cocoa
I should cocoa!
Catch Phrase. A slightly dated British exclamation meaning 'certainly not!' 

Longman's English Idioms adds a word of caution: 'This phrase is not recommended for use by the foreign student.'
But why 'cocoa'? As always when in difficulty with a phrase origin, turn to rhyming slang. 'Cocoa' is from 'coffee and cocoa', almost rhyming slang for 'I should say/think so!' Often used ironically. Current by 1936.
Dictionary of Popular Phrases (1990) by Nigel Rees

I should cocoa! (also I should coco!)
excl. [1930s+] you must be joking! don't make me laugh!
[I should cocoa =
I should say so; esp. popular in BBC Radio's Billy Cotton Bandshow in the 1950s]
Cassell's Rhyming Slang (2000) by Jonathon Green
"I should coco"
- the title of a Supergrass album. An interview with their drummer identifies it as rhyming slang for
"I should say so".

"The Ambivalent Vestibule":    
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