a toilet/bathroom She's gone to the loo. Can I use your loo, please? Word Origin 1940s: many theories have been put forward about the word's origin: one suggests the source is Waterloo, a trade name for iron cisterns in the early part of the century; the evidence remains inconclusive.Extra examples He flushed the letter down the loo. We got stuck in a traffic jam and I was desperate for the loo. I just need to pop to the loo. She’s gone to the loo.