someone who is suddenly quiet and meek
Idiomatic expression likening a person to a borrowed cat that is unusually timid and reserved in an unfamiliar environment. Often used when someone who is normally outgoing becomes shy or silent in a particular situation.
He's usually noisy, but today he's as quiet as a borrowed cat.
初めての会議で、彼女は借りてきた猫のようにおとなしかった。
At her first meeting, she was as meek as a borrowed cat.
The phrase literally means 'a borrowed cat'. Cats are typically cautious and reserved in unfamiliar places, so a borrowed cat taken from its home is especially quiet and timid. The expression figuratively describes a person who becomes unusually meek in an unfamiliar or formal setting.