noun
lion at home but weakling elsewhere; person who is bold only at home
Describes someone who acts tough or assertive at home or in a safe environment but becomes timid or meek outside. Often used critically or humorously.
彼は家では威張っているけど、外では陰弁慶だ。
He acts bossy at home, but outside he's a lion at home and a mouse elsewhere.
陰弁慶
I want to be confident outside too, so I don't become someone who's only bold at home.