Knockback

Knockback is a basic mechanic in Terraria, whereby players and enemies are repelled in the opposite direction of an attack. Most weapons and environment objects that cause damage also cause knockback, to varying degrees. Some enemies are immune to knockback, particularly Bosses and other enemies that travel through blocks, like the Worms and Man Eater. The Cobalt Shield grants the player knockback immunity. Knockback has the same effect as the Grappling Hook, which can be used to reach the Hermes Boots speed instantly.

The most player-induced knockback currently can be inflicted using the Thorns Potion, which has a knockback value of 10. This is followed immediately by the Enchanted Boomerang, Thorn Chakram, Flamarang, Hellfire Arrow, Grenade, and Breaker Blade, which all have a knockback value of 8. Also the added status of "Heavy", increasing the knockback values by roughly 100%, raises the knockback value of the weapons listed above to 16.

Knockback Values
 * 0: No knockback
 * 1: Very weak knockback
 * 2: Weak knockback
 * 3: Light knockback (?)
 * 4: Average knockback (?)
 * 5: High knockback (?)
 * 6: Very high knockback (?)
 * 7: Very very high knockback (?)
 * 8: Extreme knockback
 * 9+: Insane knockback

Underground

 * Giant Worm

The Corruption

 * Devourer

Underground Corruption

 * World Feeder

Jungle

 * Snatcher

Underground Jungle

 * Man Eater

Dungeon

 * Dungeon Guardian

Underworld

 * Bone Serpent

Bosses

 * Eye of Cthulhu
 * Eater of Worlds
 * Skeletron
 * Wall of Flesh
 * The Twins
 * The Destroyer
 * Skeletron Prime

History
longer immune to knockback.
 * 1.0.6: The Cursed Skull is

the wall of flesh no longer immune to knockback