What is the origin of 'Hump Day'?

[Exapno_Mapcase]

Interesting that most of early examples use “over the hump” with the phrase being shortened as it passed into common usage.

Hump is a very old word, but it apparently started taking on this sort of metaphorical usage early in the 20th century.

A meaning attested from 1901 is “mound in a railway yard over which cars must be pushed,” which may be behind the figurative sense of “critical point of an undertaking” (1914).