To give "fair warning" is to let someone know you are going to do something before you do it. In order to be classified as "fair warning", you need to give the person between 1 and 5 seconds before the action takes place. Anything occurring before or after those times would then be considered "unfair".
Jered- "Hey Marcus, I'm going to throw this ball at your nuts." (Jered waits three seconds, then throws.)
Marcus- "What the hell was that?!?
Jered- "Hey, I gave you fair warning."