further explanation

from Days of the Week:

"Wednesday is often referred to as "hump day" because of its position as the middle day of the work week. If the work week were a hill. Then Wednesday would be the crest. It is all down hill from there. (Whether the down hill ride is a coast or a descent into a swamp is left to the individual.)"

More definitions here.
And who knew there was a Lakota Sioux war chief named Hump who fought against Custer at the Battle of Little Big Horn????