It's the headache you get when your marijuana buzz starts to come down.
The only ways to cure a weedache is to either smoke more
weed or go to sleep. If you choose to smoke more
weed then later on when that buzz wears off your weedache will be twice as
bad.
One of the easiest ways to get a weedache is to get really high and then have to do something strenuous like work in the sun. Another way is to get high early in the day and then stay up all day and not get high any more, sorta similar to drinking
alcohol early in the day and then coming off that buzz.