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1.
A joke about something that should not be joked about, but is funny nonetheless.
John: Why don't Jewish cannibals like eating Germans? They give them gas!

Mike: That's hellarious dude!
by briscoeMD June 21, 2010
 
2.
Painfully funny: humorous social communications that hit home and would insult if not for a good sense of humor; introspectively painful and funny!
When they said "it's not over until the fat lady sings" We belted out auld lang syne...it was HELLARIOUS!
by tree frog January 04, 2014
 
3.
When a person from the BAY AREA finds a situation, person, or something outrageously funny, it CAN be described by this term.
At a party, a bunch of people are chattering about until they mention a really funny person they all know. One of them then says, "You know Steven? That guy's hellarious!!"

"Man when that girl didn't know what OJ meant, that was hellarious!!"
by Makola2 February 01, 2010
 
4.
adj. Something that is hellaciously funny; Very funny.
"Man that joke was hellarious"