The British-English phrase I should cocoa, also I should coco, is an expression of emphatic agreement, meaning I should say so, I should think so, I should hope so. It is also used ironically as an expression of disbelief, derision, etc., meaning I should think not.

It is generally said that, in this phrase, cocoa is rhyming slang for the adverb so in I should say so.

If this is the case, there is no truncation, contrary to the usual rhyming-slang formation (for example, scooby, rhyming slang for clue, is short for Scooby Doo, the name of a cartoon dog which features in several U.S. television series and films).

This is what the British lexicographer Jonathon Green (born 1948) explains in Cockney, published in the Oxford English Dictionary blog on Friday 17^th August 2012:

The original rhyming slang, which was a conscious attempt to mystify the uninitiated, depended on the omission of the rhyming element, for example: ‘Barnet fair’/‘hair’ (1857) to barnet (1931); ‘china plate’/‘mate’ (1880) to china (1925); ‘Hampstead Heath’/‘teeth’ (1887) to Hampsteads (1932); and ‘Sweeney Todd’/‘flying squad’ (1938) to Sweeney (1967). However this was by no means a rule, and there exist a number of terms in which the entire compound is pronounced—hence Adam-and-Eve/‘believe’ (1925), cocoa/‘say so’ (1936), or tea-leaf/‘thief’ (1903).

I should cocoa is first recorded in The Gilt Kid (Jonathan Cape – London, 1936), a novel set in London, particularly in the underworld of Soho, by the British author James Curtis (born Geoffrey Basil Maiden – 1907-77):

‘She’s got a bloke, a regular customer, get me, who pays the rent of the flat and floats in for a bit of under occasionally, but most of the time she’s on the bash round the flash bars trying to see what she can pick up on the side.’
‘Oh, I get you. On the make all the time. One of the wide girls.’
‘That’s right. Well, what with the steamer she’s got for regular and the odd ones she picks up she don’t do so bad, I should cocoa.’

The British journalist and author Gerald Gould (1885-1936) reflected on the phrase in his review of The Gilt Kid, published in The Observer (London, England) of Sunday 5^th July 1936:

How, I wonder, does Mr. Curtis know all he apparently does know about the strange speech of the underworld? These folk are stern realists—stern surrealists: they make a point of getting what they can, but also, it seems, of expressing what they can’t. “I should cocoa,” for instance. Only the context helps me to suppose that this means: “I should suppose.” At least, that is what I cocoa it means.

I have found two instances of I should cocoa in Good Clean Boy, a short story by the British author and screenwriter Robert Westerby (1909-68), published in The Bystander (London, England) of Wednesday 23^rd February 1938. The first instance is used in the sense of I should say so, the second in the dismissive sense of I should think not. Jack, a seventeen-year-old boxer, is walking along a street with Lou, his manager, to meet a man named Levy:

“Where we meetin’ ’im?” Jack said.
“At the caff on the corner White Street.”
“What ’s ’e want, eh?”
“Talk business.”
“’Oo wiv? Me?”
“Why not? You ’re in business, ain’t you?”
“Me? I should cocoa! I’m a boxer, ain’t I”
“Werl?”
“Oh, I dunno. I don’t understand all this mucking abaht.”

When Jack meets Levy, the latter tells the former to lose his fight against a boxer named Charley Wilks; Jack replies:

“Why doncher [= don’t you] back me to lick Wilks, then? It ’s a push-over—a lay-down.”
“At three to one on? I should cocoa! Talk sense, kid. I ’m tryin’ ter show yer how to be smart.”
“You mean three to one against Wilky?”
“Yerce. Get it now? You let me play your bit, and you ’ll clean when we cleans. Come in and be a sensible kid. It ’s just business, see?”

In the comic strip Andy Capp, by the British cartoonist Reg Smythe (born Reginald Smyth – 1917-98), published in the Daily Mirror (London, England) of Thursday 29^th December 1960, the rent collector uses the phrase in the sense I should think not:

– Andy: Come inside, lad, an’ ’ave a game o’ cards till the rain stops –
– rent collector: I musn’t, but thanks all the same!
(to himself) I should cocoa! – I’d rather carry on an’ get wet than go inside an’ get soaked!

 

The Daily Mirror (London, England) of Monday 12^th April 1971 used we should cocoa in the sense of we should think not in a comment on a letter by one reader named Christine:

Now that the unit ten is so much in vogue, why not do away with those superstitious Anglo-Saxon sevens and twelves completely? I mean, let’s have ten months in the year for a start, ten days in a week, and adjust the sun and moon accordingly.
[…]
Also, if there were only twenty hours in the day instead of twenty-four, lees production would be lost through strikes as the striking day would be shorter.
◊ And after 2 p.m. our lad George would be putting in for overtime—we should cocoa!

In the first line of Getting ready to meet my public red in nose, bald on top, published in the Liverpool Echo (Liverpool, Lancashire, England) of Thursday 19^th April 1984, John Davidson punned on the phrase when telling how he appeared as Jo-Jo the clown during a performance of Billy Smart’s Circus:

Send in the clowns—I should Co-Co . . .
It was the day your long-suffering theatre critic took to the stage or rather, the sawdust ring. Red-nosed, unpaid and unprepared.

I Should Coco (Parlophone – May 1995) is the title of the debut studio album of the British rock band Supergrass.