What determines an "outplay"?

Predicting what they will do, and playing based on that prediction, and then being right and getting an advantage on them because of it.

being disadvantaged

Outplaying someone generally means you're in a disadvantaged situation (can be lots of things, lower HP, outnumbered, or against a counterpick) and you come out with a tangible advantage (a kill, wasting the enemy's time by surviving, or blowing enemy summoner spells being common advantages).

Basically when you are fed/talk shit and you get owned by an underdog player in the game by a far shot.

Walking away from Mordekaiser