Poison Knife is Slinger Ammo in Monster Hunter World (MHW).
A knife coated with poison that can be shot from your slinger.
Throwing Knives, Paralysis Knives, Poison Knives, and Sleep Knives are all found as pickups left behind by fleeing Gajalaka. Additionally, Knives of all kinds are found in the Supply Box on a handful of Quests. Each kind of Knife can be crafted by combining a normal Throwing Knife with a Parashroom, Toadstool, Sleep Herb, or Tranq Bomb.
Note that all Throwing Knives count as Supply Items, and will therefore be removed from a Hunter's Item Pouch upon returning from the field.
Slinger Capacity doesn't affect Throwing Knifes.
See Slinger for more information about slinger ammo.