under the weather

Ailing, ill; also, suffering from a hangover. For example, She said she was under the weather and couldn't make it to the meeting. This expression presumably alludes to the influence of the weather on one's health. [Early 1800s] The same term is sometimes used as a euphemism for being drunk, as in After four drinks, Ellen was a bit under the weather.

The adjective has one meaning:

Meaning #1: somewhat ill or prone to illness
  Synonyms: ailing, indisposed, peaked, poorly, sickly, unwell