Quickly lock the door before something unwanted gets in (or out).
When the intranet police come to your office, it's Katy bar the door.
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.