he, she, etc. wouldn't say boo to a goose

he, she, etc. wouldn’t say ˌboo to a ˈgoose

(informal) somebody is very shy and afraid of upsetting or annoying people: How could he ever succeed in politics? He wouldn’t say boo to a goose.
People shout Boo! when they want to surprise or frighten somebody.
See also: boo, goose, say