What does Brown bag fever mean?

Brown bag fever meaning in Urban Dictionary

basically a more widely acknowledged term for a hangover; you can come down because of the "Brown bag temperature" just after a night of drinking, for this reason the word 'brown case' had been coined refering towards the brown case that is placed over alcoholic beverages when purchased at a liqour store and 'fever' to refer into the unwell, almost fever-ish experience one gets from drinking excessively.