It's when reality defies expectation, and in fact you don't know what it meant because you didn't use it right once.
There are different definitions "an objectively or humorously sardonic utterance, disposition, quality, etc."
Also, when he called me an idiot, that did defy my expectation in that manner that he called me an idiot when I was giving him advice, and when he was an idiot himself
Irony =/= LOL SO RANDOM