A long, loose shirt worn to bed instead of pajamas.
Simple compound of 'night' (Old English 'niht') and 'shirt' (Old English 'scyrte'). The term became popular in the 19th century when nightwear became a distinct category of clothing.
Nightshirts are an ancient concept—men in medieval times slept in simple linen tunics, and women wore long shifts that's the ancestor of the modern nightshirt, making it one of the oldest forms of clothing that's still basically unchanged.
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