Weak Armor is a Pokémon ability introduced in the fifth generation. The Pokémon with this ability get faster with attacks.
Pokémon with Weak Armor increase their speed with every physical attack. In generation V and VI, every physical attack only increased speed by one stage. In Generation VII, this was improved to 2 stages. If a Pokémon with Weak Armor is hit with a physical multi-hit move, they will get speed increases for each hit.