Quicker than lager turns to piss

What's the meaning of the phrase 'Quicker than lager turns to piss'?

Very quick.

What's the origin of the phrase 'Quicker than lager turns to piss'?

A recent , that is, 1980s, expression, originating in the UK. The earliest citation that I can find of it is from John Cooper Clarke's poem Beasley Street, which was included in the collection Snap, Crackle & Bop, 1980 and the line was probably coined by him:

People turn to poison
Quick as lager turns to piss
Sweethearts are physically sick
Every time they kiss
It's a sociologist's paradise
Each day repeats
Uneasy, cheasy, greasy, queasy
...beastly, Beasley Street