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1.
The feeling you experience after a long period of smoking. Much like a hangover you would experience after a long period of drinking. Symptoms include loss of motivation, an overwhelming urge to eat everything in site, temporary body paralyzation where you are stuck on a couch or in a bed for a long period of time, excessive sleeping, short term memory loss.
"Dude, I got so stoned off those bong rips from a couple hours ago now I have a killer hashover. Bring me some treats, I can't move!"

or

"Man, I really wish we wouldn't have smoked 20 bowls last night. I woke up with a killer hashover and forgot I had an exam at 8am."
by JSimpsonBMcintyre January 25, 2009
 
2.
It is the same as "hash out" and it means to review or to go over something once again.
"Let's hash over the suggested solution!"
by narvan March 21, 2009