Meaning: Quickly lock the door before something unwanted gets in (or out).
Example: When the tax man comes to call it's Katy bar the door.
Origin: From a book of poems called "King's Tragedy" by D. G. Rossetti published in 1881. It tells of an attempt by one Catherine to save the life of Scotland's James I by throwing her arm across a doorway to bar his enemies
The phrase was further popularized in a 1980s Miller Lite Beer commercial featuring ex-baseball player Bob Eucker.
