To cause trouble, suffering, or illness to someone; third person singular present tense of 'ail.'
From Old English 'eglan' or possibly Old Norse 'eggja' (to distress). The word has always meant 'to trouble' or 'to cause suffering,' with the medical sense developing later when 'ailing' came to mean suffering from illness.
The phrase 'What ails you?' is pure old English gentility—it sounds like something from a Victorian novel, but it's actually been used for over 1,000 years because 'ail' captures both physical sickness and emotional distress in one word.
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