From Longman Dictionary of Contemporary Englishnightshirtnight‧shirt /ˈnaɪt-ʃɜːt $ -ʃɜːrt/ noun [countable] DCCa long loose shirt that someone, especially a man, wears in bed
Examples from the Corpus
nightshirt• Her sensible cotton nightshirt must have been one of his discards.• Polly turned the deadlock, and before reaching for the latch dabbed at her eyes with the hem of her nightshirt.• She got up from the bed, buttoned up her nightshirt and went to the door.• And picking up her nightshirt from under the pillow, she went into the bathroom. 