Moxie

Introduced in Generation V

This Pokémon's Attack rises one stage upon knocking out another Pokémon, even a friendly Pokémon. This ability does not take effect when the Pokémon indirectly causes another Pokémon to faint, e.g. through poison or Spikes. If this Pokémon knocks out a Pokémon with Mummy, the former's ability will change without taking effect.

“When the Pokémon knocks out a target, it shows moxie, which boosts its Attack stat.”