Green’s Dictionary of Slang

alphabet n.

1. (US prison) an extremely long sentence, in N.Z. system a life or Preventive Detention sentence [pun on SE sentence, i.e. a ‘sentence’ so long that it cannot be characterized by numbers; or the criminal has been given ‘the lot’].

[US]Other Side of the Wall: Prisoner’s Dict. July 🌐 Alphabet: A sentence so long that it cannot be characterized by numbers. (VA).
[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 6/2: alphabet, the n. 1 a life or Preventive Detention sentence. [because the criminal has received the lot].

2. (N.Z. prison) a mix of class A, class B and class C drugs.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 7/1: charged for or with the alphabet arrested for possession of Class A, Class B and Class C drugs: ‘What did they charge you with?’ ‘The alphabet’.

3. (Aus. und.) law enforcement agencies known by their initials.

[Aus]A. Nette Orphan Road 73: [L]ike-minded groups could train for the coming race war, without any prying questions from the ‘alphabets’, as he referred to law enforcement agencies.

In compounds

alphabet soup (n.)

a mass of representatives from a variety of agencies (e.g. law-enforcement) the initials of whose names make the ‘alphabet’.

[US]D. Winslow ‘Broken’ in Broken 10: Half an hour later, the boat is awash with alphabet soup.
alphabet weekend (n.)

(N.Z. drugs) a weekend almost entirely devoted to wide-spectrum drug use.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 7/1: ‘I had an alphabet weekend: A, B, C, D, E and F.’ i.e. the speaker took a Class A drug (e.g. LSD); a Class B drug (e.g. speed); and a Class C drug (e.g. cannabis); got drunk (D); took Ecstasy (cf. E) and ‘got Fucked’ (he either had sex, or became physically and mentally incapacitated.