Wouldn't say boo to a goose

Origin

Imagine a common farm goose: large, loud, and known for its assertive hissing and flapping, especially when protecting its territory or young. To say 'boo' to such a bird is a minimal, almost childish attempt to startle or challenge it. The phrase emerged, likely in the 19th century, to paint a vivid picture of extreme timidity. It suggests a person so utterly unassertive and easily intimidated that they lack the courage to even make a small, harmless sound to a creature typically considered a minor, if noisy, nuisance, let alone stand up to a real threat. It’s a wonderfully graphic way to describe someone who is a shrinking violet in every sense.