A variant or archaic spelling of 'hair', or possibly a type of coarse fabric used historically in religious garments for penance.
From Old English 'hær', 'haire' appears as both a spelling variant and potentially as a specialized term for a rough hair-cloth ('haircloth') worn by monks and penitents in medieval times.
Religious monks wore 'haires' (rough haircloth) next to their skin as self-punishment to atone for sins—it was intentionally scratchy and uncomfortable, which was kind of the whole point!
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