unwell
adj.feeling sick or not healthy. You use this word to describe someone who is ill, but it often sounds a bit more polite or formal than saying 'sick'.
adj.suffering from illness or poor health. Often used predicatively following linking verbs such as 'feel', 'look', or 'seem'.
“He stayed home today because he felt unwell.”
“The teacher noticed that Sarah looked quite unwell and suggested she visit the school nurse.”
“Although she appeared physically recovered, she continued to feel generally unwell for several weeks following the initial infection.”
Typically used predicatively (after the verb) rather than attributively (before the noun).
In modern English, 'unwell' is almost always used after a verb like 'is' or 'feels' rather than directly before a noun.
From un- + well.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.