To ail means to suffer from illness or poor health. When something ails you, it causes physical discomfort or sickness. A doctor might ask, “What ails you?” meaning “What's making you feel unwell?”

You might say “My stomach has been ailing me all day” or hear someone wonder “What ails that plant?” when its leaves turn yellow. The word works for bodies, minds, and even things like struggling businesses or broken machines.

In older stories, you'll often find characters asking “What ails thee?” which sounds formal but simply means “What's wrong?” The word ailment refers to a specific illness or condition: a minor ailment might be a headache, while a serious ailment could require medical treatment.

Though ail sounds a bit old-fashioned compared to saying someone is “sick” or “ill,” it's still useful when you want to describe ongoing discomfort rather than a sudden illness. When something ails a person, creature, or thing, it troubles them in a persistent, nagging way.