My sister came up with this word to describe me when she was about eight years old. It is a portmanteau combining "hilarious" and "comedian." It is used sarcastically conveying the idea that "although you think you're funny, you're not."
Dude: Jack be not nimble; Jack be not quick; Jack couldn't jump over that therecandlestick.
Bro: Hilarious, dude.
Dude: I'm a poet, I knows it.
Bro: Yeah, you're a real hilarian.
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'"