A Magic the Gathering term referring to the imaginary world where your shitty deck draws everything it needs in order to actually function, your opponent draws 0 interaction, or your new janky spell from Urza's Saga you proxied a day ago actually impacts the game instead of being dead in your hand. Compare to reality, where you mulligan down to 5 and scoop on turn 3, having accomplished nothing. It's the impossible hypothetical, like winning the lotto or Magic the Gathering players ever losing their virginity.
One with Nothing as a madness outlet only works in Magical Christmas Land where every other better discard outlet doesn't exist and you have the mana to do something with your madness cards.