What's a Pea Souper?

Pea soup fog is a very thick and often yellowish, greenish or blackish fog caused by air pollution that contains soot particulates and the poisonous gas sulphur dioxide.

What is meant by a pea souper?

British, old-fashioned + informal. : a very heavy and thick fog The fog was very bad—a real pea-souper.

Where does the saying pea souper come from?

On several occasions, people fell in the Thames and drowned because they could not see the river right in front of them. And so, for obvious reasons, the thick London smog became known as a 'pea souper'.