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!!"
A 'better' alternative to using the word 'crazy' or 'insane' to someone. Or just a fun way to call someone crazy. Also, 'mentally jolly' can be used. From the Family Guy episode 'The Fat Guy Strangler'.
"Dr, is my brother really insane?"
"Mrs Griffin, we don't use the word 'insane', we prefer the term 'mentally hilarious'"
And dying in yo' bed many years from now, would ya be willing ta trade all da days from dis here day ta dat fo' one chance, just one chance, ta come back here as young men an' tell our enemies dat dey may take our lives but dey will never take our freedom ... YUB NUB!