A “pea-souper” is a thick fog, often with a yellow or black tinge, caused by air pollution.

The idiom was first used to describe the thick, choking smogs that settled over London, caused by lots of people burning fossil fuels in a close vicinity, as early as 1200. The smogs were compared to pea soup due to their colour and density.

“Be careful when you’re driving — it’s a pea-souper out there.”