“落汤鸡”，中文俗语，字意为落水或浑身湿透的鸡（a drowned chicken；a chicken in the soup），现在形容被雨水浇得浑身湿透（be caught in the rain and soaked through），极其狼狈的样子。
与英文短语“be like a drowned rat”意思相近，表示“to be very wet, especially because you have been in heavy rain”。
My goodness, you're soaked through. Where's your car?
He was wet all over like a drowned rat.