Phailus is a portmanteau of fail and phallus. It is used to describe a situation where one is dicked due to failure. It can also describe what a man has after he has been cock-blocked or what he has if he fails to perform in bed.
It is similar to phail.
"Dude, I was totally gonna hit that before her friend dragged her away."
"Hey, at least least I didn't have a phailus last night with that one chick."
"No cool dude, not cool."