Hairshirt.

Hairshirt [wiki], as the name implies, got its name because it is a shirt made from animal hair (now it means object that is worn to induce discomfort or pain). Who would wear such a shirt? mental_floss explains:

Hairshirts used to be the clothing article de rigueur for penance and mortification. [...] Beyond their famous use by ascetics, lay persons also wore them periodically to stave off the corporeal temptations, and men of note sometimes wore them under their robes. Today only the Carthusians and Carmelites can be seen wearing them

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