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'"