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1.
When you stay up into the early hours of the morning playing Halo, then feel like crap the next day.
"I finally beat Halo 3 on LEGENDARY, but now I have and awful Halo hangover.
by Kowdee December 23, 2008
 
2.
The headache you get after staying up all night playing xbox or other game system.
"I was playin xbox till 4 AM and then woke up at 9 with a halo-hangover"
by Country Boy Jim October 29, 2009
 
3.
The result of a late night of extremely intense Halo playing.

Symptoms include, but are not limited to: headache, drowsiness, nausea, bloodshot eyes, and sniper paranoia.
"Ugh, I'll never play Halo again; I'm paying for it with Halo Hangover."

"I'm so sleepy. I haven't had Halo Hangover in a while."
by Adaptoid April 11, 2007