The pleasant feeling of still being drunk when you wake up the next day after going to bed/passing out after a night of drinking. Far better than the alternative (cf hungover)
Woke up pleasantly surprised after last night's shenanigans to find I was leftover drunk rather than nursing a horrific hangover.
Leftover drunk is a special kind of drunk. You wake up, the day just dawning. After last night, you expected to be spending a miserable day in bed/on the couch/under a highway overpass, but instead you feel great. Music sounds and food tastes even better than you remember. You feel like you've cheated Death itself.