Region: General | Usage example: Installing security after the data leak felt like locking the barn door.

Lock the barn door

Meaning

Take action after the damage has already been done.

Used to warn against late responses to preventable disasters.

Origin notes can vary by region and source; this page gives the commonly reported explanation and usage context.

Farmers realized locks were useless after animals had escaped.

Appears in 19th-century farming manuals and cautionary tales.