What does "wouldn't say boo to a goose" mean — and why is it funny?

informal, British

Meaning

Describes a person who is extremely shy, meek, or timid.

Where it comes from

An old British saying. Geese are loud, aggressive birds, so the meekest possible person is one who would not even dare to startle one.

Why it is funny

The comedy is the gentle picture of cowardice. Saying 'boo' is the smallest act of boldness imaginable, and the phrase finds someone too timid even for that — outmatched by a goose.

Used in a sentence

"Don't worry about him causing trouble — he wouldn't say boo to a goose."