dead weight
noun [ C usually singular ] uk/ˌded ˈweɪt/ us/ˌded ˈweɪt/
难以搬动的重物;沉重的人（或物）
the heaviness of a person or object that cannot or does not move by itself
She may be small but, when I have to carry her upstairs after she's fallen asleep, she's a dead weight (US also she is dead weight). 她个子虽小，可当她睡着后我不得不把她抱上楼时才发现她重得要命。