Definition & Meaning of "{not} [say] boo to a goose"

What is the origin of the idiom "not say boo to a goose" and when to use it?

The idiom "not say boo to a goose" is believed to have originated in Britain. The phrase originally referred to someone who was so timid or shy that they wouldn't even make a noise like "boo" to scare away a goose. It is used to describe someone who is exceptionally shy, timid, or introverted. It also suggests that the person is unlikely to speak up or take any action that might cause even a slight disturbance.