The story
From the traditional opening hours of banks in the 1800s, and in some parts of the United States, maintained until the 1960s.
Etymology from Wiktionary, CC BY-SA 4.0.
bankers' hours
Today it means: The time period between 10 a.m. and 3 p.m. of a weekday (corresponding to a late start, an early quit, and no weekend work). Full definition →