The phrase under the weather began as a literal image from seafaring language, where bad weather could make a voyage difficult or dangerous. It suggests a person or situation exposed to rough conditions and feeling the effects of them. That concrete picture still helps explain why the expression later came to describe a person who feels slightly ill.

In modern English, under the weather means feeling a little unwell, tired, or off your usual form. It is used for mild sickness rather than serious illness or emergencies, and it often appears with the verb be. Learners often meet related everyday expressions such as Break the Ice, Piece of Cake, and Blue Moon Idiom in the same kind of informal conversation.

Word or Phrase	Definition	Example
Under the weather	A phrase meaning slightly ill or not feeling well.	I am under the weather, so I may leave early.
Feel off	An informal phrase meaning not quite well.	She feels off today, so she is resting.
Out of sorts	A phrase meaning slightly unwell or emotionally unsettled.	He is out of sorts after a long week.
Feeling unwell	A formal alternative meaning not feeling healthy.	I am feeling unwell, so I will stay home.
Slightly indisposed	A formal phrase meaning mildly unwell.	The manager is slightly indisposed today.

The common pattern is be under the weather, and contractions are natural in speech and casual writing, as in I am or I am. The phrase is informal and fits conversation, emails, small talk, and brief apologies for missing work or social plans. In highly formal medical writing, clinical language is clearer, and in both the UK and the US the idiom is widely understood with no major regional difference.

Idea	Example
Use be plus the phrase.	I am under the weather today.
Use it in informal conversation.	I am under the weather, so I will skip lunch.
Use it in casual emails.	I am under the weather and need to work from home.
Use it to explain an absence.	I stayed home because I was under the weather.
Use it in both UK and US English.	This phrase is common in both London and New York.

Several expressions overlap with under the weather, but they are not exact matches in tone or emphasis. Feeling off and out of sorts are close informal alternatives, while feeling unwell and slightly indisposed are safer formal choices. A phrase such as under the gun means under pressure, not sick, so it belongs to a different meaning entirely.

Word or Phrase	Definition	Example
Feeling off	An informal phrase meaning not quite normal or well.	She is feeling off, so she skipped the meeting.
Out of sorts	An informal phrase meaning slightly unwell or not in the usual mood.	He was out of sorts after traveling.
Feeling unwell	A neutral formal phrase for being sick.	I am feeling unwell and need a doctor.
Slightly indisposed	A polite formal phrase for mild illness.	She was slightly indisposed and could not attend.
Under the gun	A phrase meaning under pressure or facing a deadline.	I am under the gun at work this week.

In conversation, the phrase often appears in short, polite explanations, especially when someone is excusing themselves or softening bad news. It sounds natural in friendly talk, quick messages, and everyday workplace communication, where it signals mild illness without sounding dramatic. The same meaning can be expressed in a more formal way when the setting asks for extra distance or precision.

Idea	Example
Polite excuse for absence.	I am under the weather, so I will not come in today.
Casual explanation in conversation.	I feel under the weather, so I am going home early.
Friendly email wording.	I am a bit under the weather and will reply tomorrow.
Small talk with a gentle tone.	I have been under the weather, but I am better now.
Mild sickness, not an emergency.	He is under the weather and resting at home.

Under the weather is best understood as a mild, informal way to say that someone feels slightly ill, with a vivid origin in weather and sailing. It is common in both UK and US English, especially in conversation and everyday writing, and it usually appears in the pattern be under the weather. When a situation needs more formality or medical precision, phrases like feeling unwell or slightly indisposed communicate the same idea more appropriately.