One over the eight
The final drink that renders someone drunk.
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:
See also one for the road.
Other 'One' phrases: