Many times when partying hard, a partier will drink enough alcohol to be drunk for many more hours than he/she is able to stay awake. So, when he/she wakes up in the morning, the first thing he/she does is looks around, and goes, 'Man, I'm still drunk.' Thus, when waking up from a hard night of partying still drunk, you are 'stilldrunk.'
Girl #1, "Hahaha! I can't believe I'm still drunk!"
Girl #2, "Honey, you're not 'still drunk,' you're STILLDRUNK!!!"