A hangunder would be the opposite of a hangover. If a hangover is a headache because of the effects of alcohol, then a hangunder would be a headache because of a lack of alcohol.

Never, ever, ever go drinking with a migraine. While it appears to go away, it is just hiding under a lovey blanket of alcohol. When you stop drinking it will come back, and now you're too drunk to deal with it sensibly (and have too much blood alcohol to take aspirin or paracetamol, unless you want to add liver failure to your list of woes).

Trying to go to sleep, you will experience the true horror of a hangunder. Not only will your head will feel several time more evil than when you set out, but you will also be tired and dehydrated - sensations which are not appreciated when the room is spinning, you can't see properly, can't think straight and have no real concept of time anymore.

And then in the morning you'll have a hangover to boot. Lovely!