under the weather
Feeling sick; ill. It can also mean that a person is feeling sad or depressed.
in the old days, when a sailor was feeling seasick, "he was sent down below to help his reocvery, under the deck and away from the weather."
来源是这样子，以前在航海时，当水手晕船时，他就会躲到甲板，这样就远离糟糕的天气，所以用under the weather 来指代心情不好，或感觉不舒服；
He cannot come to the class today because he is under the weather.
Jessica just got back from her vacation in Florida, but sadly, she returned feeling a bit under the weather because she had a sore throat, a runny nose, and a fever.