Under the weather means feeling slightly ill. A bit under the weather, feeling under the weather, still under the weather.
It is deliberately mild and is what people say to explain an absence without giving details. The phrase is nautical, from a sick passenger sent below deck away from the weather side of the ship.
1phraseunder the weatherFeeling slightly ill.
“I'm feeling a bit under the weather, so I'll stay home and rest today.”
under the weather is a state.
From the nautical under the weather bow, the exposed side of a ship, where a seasick passenger was worst off.
Built from
From under the weather bow (“affected by bad weather; seasick”); weather bow is a nautical term referring to the side of a ship exposed to bad weather.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms