Someone who is insecure (not confident) or not bold would never speak strongly in a way that would shock or command someone to do something. This is an old idiom originating in the UK. Anybody who shouts loudly and firmly at geese can intimidate them; indeed minding geese was traditionally a job for small girls. Anybody who doesn't dare say "Boo!" to a goose is too timid to control a gaggle (group) of geese. from: http://idioms.thefreedictionary.com/wouldn't+say+boo+to+a+goose Wouldn't say boo to a goose (British informal) also wouldn't say boo to a fly (American/Australian informal/informal). If someone wouldn't say boo to a goose or a fy, they are shy and nervous, e.g. "She wouldn't say boo to a goose, so I don't think she's cut out for a career in the police or as a military instructor or a prison guard or a strict disciplinarian."

Hi there, I like such questions related to English expressions and idioms, I learn from your questions lol, not answering u! ''Wouldn't say a boo to a goose'' is a British informal, also wouldn't say boo/wouldn't say boo to a fly (American informal) if someone wouldn't say boo to a goose, they are shy and nervous. Also describing a quiet, exceptionally shy person

She is very timid and meek.