sick as a dog 病得非常严重；病得厉害；病的十分严重

【例句】
She was feeling so bad, she was as sick as a dog.
她感觉一点都不好，因为她病得厉害。

Yesterday I was sick as a dog with the flu. I stayed in bed all day.
昨天因为感冒我病得十分严重，整天都在床上躺着。