One of the mysteries of the English language finally explained.
With reference to a sermon by or letter from an ecclesiastical authority: (addressed) to the clergy.
Mid 16th century; earliest use found in Nicholas Ridley (c1502–1555), bishop of London and protestant martyr. From post-classical Latin ad clerum from classical Latin ad to + post-classical Latin clerum, accusative of clerus the clergy.
ad clerum/ˌad ˈklɛrəm/
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