say boo to a goose

say boo to a goose (third-person singular simple present says boo to a goose, present participle saying boo to a goose, simple past and past participle said boo to a goose)

1. (chiefly in the negative) To have courage and be assertive.

This phrase is chiefly used in the negative, and in conditional constructs. For example, she wouldn't (or couldn't) say boo to a goose = she is very timid.