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.
"I wake and baked like 5 hours ago and now I have this awful weedache."
"I got high before I went to go pull shingles and that midday sun gave me a weedache."
"After ripping that G-bong a couple times my head don't hurt anymore."