Where to find and use the Dirty Key in Nioh 3
Reporting by Pcgamer indicates the Dirty Key in Nioh 3 is obtained by defeating a Yamainu in the ruined village early in the game.
The Yamainu patrols the village you reach from the Prospect Hill shrine: head down the path past two yokai and you'll see the wolf walking up and down. Make sure to deal with the enemy on the right-side rooftop who throws stuff before you engage the Yamainu.
Yamainu can be softened with projectiles or by drop-attacking from rooftops. They also get stuck on scenery and struggle to enter buildings, which lets you cheese the fight. After you defeat it you receive the Dirty Key and some dialogue plays.
Use the Dirty Key on the easternmost house in the ruined village: enter from the Prospect Hill shrine, go down the street, turn left, then left again up the steps to the house with a cooking pot and a Chijiko above. Interact with the door to open the chest inside and claim assorted gear and the Armor Piercer Ninjutsu Secret Skill Text.