One over the eight

Meaning

The final drink that renders someone drunk.

Origin

This originated as UK military slang. The first reference to it in print is in Fraser and Gibbons' Soldier & Sailor Words, 1925:

"One over the eight, one drink too many. Slightly intoxicated, the presumption being that an average 'moderate' man can safely drink eight glasses of beer."

After a gallon of beer 'slightly intoxicated' seems a bit of an understatement. Beer was weaker in the 1920s than it is now, but even so.

Other phrases first cited in Fraser and Gibbons:

Go Dolally
Jam tomorrow
Loaf of bread (head)

See also one for the road.