Hump, in its colloquial meaning, refers to the most difficult part of something. As in ‘We are over the hump’ (Ya hemos pasado lo peor). Traditionally, it means in Spanish “joroba, giba o montículo”.
Hump Day is a slang expression that refers to the middle of a work week (Wednesday). It implies that the hardest part of the week is gone, and the weekend starts to approach.
You can also find the expression: Happy hump day! (feliz mitad de semana! o feliz miércoles!)… -By the way, why didn’t I post this on a Wednesday??? 🙂