Anti-Splashable refers to a card that will only see use in very specific decks. These cards are neither Splashable or Semi-Splashable because of how inflexible they are. Anti-Splashable cards generally apply to only decks that focus on a specific card or a specific Archetype.
For example, "Bottomless Trap Hole" is a very Splashable card because it can be used in almost any deck a duelist decides to create, and "Dragon's Rage" is Semi-Splashable because it will function in almost any deck dedicated to Dragons, but "Red Dragon Archfiend/Assault Mode" is Anti-Splashable because it will only ever be included in a deck that includes at least one "Red Dragon Archfiend".
Another example is how "Monster Reborn" is Splashable because it works in most any deck it is put in, and "The Warrior Returning Alive" is Semi-Splashable because it works in nearly any Warrior-themed deck it is put in, while "Elemental Recharge" is Anti-Splashable because it will only see use in a deck dedicated mostly to the Elemental HERO archetype.