A hangunder is defined as suffering all the ill-effects of alcohol you actually haven't got round to drinking yet. This is most pronouced with the drink made from the reannual plants, such as Ghlen Livid. If Pteppic had been able to think a little straighter when he did his Final Run with a stinking headache and a sense of general ill-being, he would have realised that he was destined to survive the Run and live to drink some really good reannual Counterwise wine shortly afterwards.
Nobody knows what happens if you have the hangunder and are then prevented from drinking the wine a day or two later. Maybe the universe explodes and re-invents itself.