There are 2 definitions for this:
1) When a complete dickhead tries to play an April Fool's joke on you after midday on the 1st April, or any other time of the year. This becomes obvious when the dickhead says 'April Fools!' after trying to trick you.
2) Anyone that tries to play an April Fool's joke on you, but you see right through it, and to top things off, they say 'April Fools!' anyway, which is just dumb.
1) Dickhead: Hey Chris, you dropped your ipod?
Chris: Did I? *Looks back*
Dickhead: *Slaps Chris over the back of the head* April Fools!
Chris: It's May, dumbass. God, what an April Fail...
2) On April the 1st, this dude comes up to me and tells me my bag's open. Being cautious, I know it's an April Fools joke and just sneer at him. Then, he shouts 'April Fool!' and starts laughing like a retard. That guy's an April Fail!