lion at home, mouse abroad
Describes a person who is domineering or boastful at home but timid and reserved outside. Often used critically or humorously.
He's a lion at home and a mouse abroad: bossy at home but meek outside.
うちの子は内弁慶で、学校では全然しゃべらない。
My child is a home tyrant; at school they don't talk at all.