A young male singer in a church choir, or someone who appears innocent but might not be.
From 'choir' (group of singers) + 'boy.' The phrase 'alter boy' and 'choirboy' became associated with innocence and purity, but in modern usage, 'he's no choirboy' suggests hidden guilt or misbehavior.
It's fascinating how 'choirboy' went from a literal job description to an ironic phrase meaning the opposite of innocent—now when someone says 'he's no choirboy,' everyone understands it means he's probably got a sketchy past, showing how context completely flips a word's meaning.
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