Can't say boo to a goose
To be so timid that one cannot even make the mildest protest,
however badly one is treated.
I was surpised to see Joe so talkative at the party.
Usually he can't say boo to a goose
Further example sentences:
She wouldn't say boo to a goose,
so I don't think she's cut out
for a career in the police. I remember her as a quiet little girl who wouldn't say boo.
She's a quiet little kid. Can't say boo to a goose.