ACAB, or Acab, is a British- and Irish-English acronym from the phrase all coppers are bastards.

This acronym is customarily written (tattooed in particular) rather than spoken.

The earliest occurrences that I have found are from the Daily Mirror (London, England) of Wednesday 20^th May 1970—Hants is the abbreviated form of Hampshire:

ACAB
The four-letter word that landed a teenager in court yesterday

A four-letter word was embroidered on a youth’s denim jacket. The word was “Acab.” It meant “All coppers are bastards,” a court heard yesterday.
The youth, Graham Van Hovston Johnson, 17, said he thought the letters meant “All Canadians are bums.”
Johnson told magistrates at Havant, Hants: “I sewed the letters on. I copied it from a Hell’s Angel down from London. He told me it means: ‘All Canadians are bums.’
“I didn’t know it referred to coppers.”
Johnson, of Dunsbury-way, Leigh Park, Havant, denied wearing R A F uniform when not in the forces, and displaying insulting letters likely to cause a breach of the peace.
Sergeant Richard Ostler said Johnson told him: “They all wear it, because they’re fed up with coppers pushing them around.”
Johnson said in court he was fed up with coppers.
“I’m always being checked because I’m out late at night and when you wear a leather jacket and have long hair they think you’re a criminal or something,” he said.
He was fined £5 on each offence.

Referring to this specific court-case in Some Excuses Seem Logical; Others Don’t Make Sense, published in The Arizona Daily Star (Tucson, Arizona) of Sunday 9^th September 1973, Murray Sinclair wrote:

Possibly it would have been all right to insult Canadians, but not the police.

The second-earliest occurrence of ACAB that I have found is from the account of the trial of Anthony Charles Jeffs, aged 21, charged with the murder of Police Constable Peter Charles Guthrie, aged 22—account published in The Birmingham Post (Birmingham, Warwickshire, England) of Tuesday 7^th November 1972 (Mr. Michael Davies, QC [i.e., Queen’s Counsel], was prosecuting):

While Mr. Jeffs was being interviewed, Mr. Davies said, Det. Supt. Thomas Jeffen noticed a tattoo on the back of his hand saying “ACAB Tony.”
The superintendent had said: “That means ‘All Coppers Are Bastards.’ Do you hate us that much?”
Mr. Jeffs, said Mr. Davies, replied: “Well, they have a job to do. I didn’t like them when I put it on. It was a fair bit ago.”

Paddy O’Gorman put the ACAB tattoos into a sociological perspective in Poor prospect of the welfare class getting equal chances, published in the Evening Herald (Dublin, County Dublin, Ireland) of Wednesday 10^th November 1999:

Poor people look different. It’s a look that’s impossible to define but possible to recognise.
And it’s not easy to describe but I’ll have a go.
Poor people have a distinctive facial expression that marks them apart. They look less healthy than others for the obvious reason that they are less healthy.
They have a care-worn and prematurely-aged look. Their faces are lined and their teeth are in poor condition.
They smoke. They are far more likely than others to have tattoos, often blue ink, amateur tattoos that they applied when they were fifteen and foolish. The tattoos may be four dots or ACAB (that’s All Coppers Are Bastards) on the fingers.
Worst of all is the so-called borstal ¹ spot—that’s a blue dot on the cheek. The borstal spot is the welfare class equivalent of the old school tie.
You will find a lot more tattoos in Ballymun ² than you will in Glasnevin or Clontarf ³.
Apartheid South Africa had a literally black and white distinction between rich and poor.
In Ireland, as in the western world generally, we have a more subtle apartheid system based on the visual clues that I have tried to describe.

¹ The noun borstal designates a custodial institution for young offenders—from Borstal, a village near Rochester in Kent, a county on the south-eastern coast of England, where the first of these institutions was established in 1901—cf. Borstal Boy (Hutchinson & Co. Ltd – London, 1958), an autobiographical novel set in a borstal, by the Irish poet, short story writer, novelist and playwright Brendan Behan (1923-1964).
² Ballymun is an outer suburb of Dublin, at the northern edge of the Northside.
³ Glasnevin and Clontarf are residential areas on the Northside of Dublin.

A.C.A.B. is one of the songs from The Good the Bad & the 4 Skins (1982), an album by the English Oi! ⁴ punk-rock group The 4-Skins—the initial letters are pronounced separately:

– in the spoken introduction:

A.C.A.B. all coppers are bastards

– in the chorus:

A.C.A.B., A.C.A.B., A.C.A.B.
All cops are bastards

⁴ The name Oi! denotes a form of punk-rock music—for more information, cf. Misunderstood or hateful? Oi!’s rise and fall, by Alexis Petridis, published in The Guardian (London and Manchester, England) of Thursday 18^th March 2010.