Lucksack is a derogative term for a favorable play that happens by chance, potentially ending with a victory due to luckily drawing a broken card. Examples of these cards include "Snatch Steal", "Fiber Jar", "Pot of Greed", "Ring of Destruction", topdecking an "Infernity Archfiend" (while you have no cards in your hand and a full Graveyard), "Heavy Storm", "Dark Hole", "Card Destruction" (in Dark World decks primarily), "Mind Control"/"Brain Control"/"Change of Heart", "Monster Reborn" and "Chaos Emperor Dragon - Envoy of the End". Usually, people discuss cards that lucksack because cards have given less experienced players comebacks or wins. As such, many of these cards are Limited or Forbidden.
Another example of a way someone may "lucksack" their opponent is by siding in a card that completely messes up the opponent, such as "Royal Decree", "Dimensional Fissure", or "Rivalry of Warlords" and drawing it opening hand both games 2 and 3 while the opponent did not draw any outs to it.
In some video games, the Destiny Draw, allowed players to draw "lucksack" cards when their Life Points fall under 4000 and are at least 2000 lower than their opponent's.