Green’s Dictionary of Slang

max n.

also all max, maxamus, maximus, maxy, old max
[all abbr. of SE maximum]

1. [early 18C–19C] gin, esp. high-quality gin.

2. [mid-19C] (UK Und.) any form of alcohol.

3. [mid-19C+] (US campus) the maximum score or achievement in an examination, the student who achieves this.

4. [1950s+] (US Und.) the maximum sentence for an offence.

5. [1960s+] (Can./US) a maximum security jail.

6. [1970s+] (US campus) the highest level or degree.

7. [2000s] (drugs) gamma hydroxy butyrate, GBH, dissolved in water and mixed with amphetamines.

In derivatives

maxy (adj.)

[mid-19C] drunk.

In compounds

max-ken (n.) [ken n.1 (2)]

[mid-18C] (UK Und.) a tavern specializing in gin.

In phrases

to the max (adv.) (also to the maxi) [SE maximum, orig. in the California youth cultures]

[1970s+] to the greatest extent, to the most extreme degree.