1. If you do not succeed at a goal that you were going for
2. If you get a low test score
3. If you are a faggy fuckfaced shithole eating anal grease sucking buttfucking shit raping douche-waffle-mc'cunt-nugget, then you can use this to describe your friends if they do something that they weren't even trying to succeed at
4. A person disses you and you are such a fuckhole that you can't think of a diss you may use "fail" here.
1. I failed at getting the job that I wanted.
2. I failed on the test today.
3. Kid accidentally trips
Faggy douchebag shitfucker: You Fail!
4. Kid calls you an inside-out asshole licker
Inside-out asshole licker: You Fail!
* Please if you really want to be a good friend tell your retarded shit-eating friends to stop saying fail, seriously nearly every one of my friends say this and I hate calling them shit fuckers, but I have to so someday they will get some sense and look back at their retardedness and say "Why did I use to say fail, God I was a regurgitating asshole"