Weak Armor

Raises Speed and lowers Defense by one stage each upon being hit by a physical move.

Full effect

Whenever a physical move hits this Pokémon, its Speed rises one stage and its Defense falls one stage. This ability triggers on every hit of a multiple-hit move.

In game description

The Pokémon's Defense stat is lowered when it takes damage from physical moves, but its Speed stat is sharply boosted.

Pokemon with Weak Armor

Using Weak Armor in Cobblemon

Abilities are passive and always active, so Weak Armor costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.

See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.