A home gym setup, generally used by cyclists, but also popular with other disciplines. It refers to a dedicated area, generally in a basement (hence, "cave") where long, grueling exercise sessions take place.
I did a triathlon in May, which meant lots of long days in the Pain Cave, since it was too cold to train outside.
A mentally and/or physically debilitating condition worse than a hang over usually brought on by the consumption of copious amounts of alcohol combined with severe lack of sleep.