A hair shirt is as it sounds: a shirt made out of hair. Though they are rarely used today, historically people in some Christian religious orders wore them as a means of penance. The shirts were originally woven with goat hair and were worn next to the skin to keep the wearer in constant discomfort and awareness of the shirt’s presence. (The shirts evolved to contain bits of metal woven with hair. Delightful.)
Here are some examples of hair shirt as a noun:
Uncle Harvey is such a hair shirt. I would rather drink soup from a toilet than listen to another of his “olden days” stories.
Merv thought yoga was a hair shirt until he tried it and enjoyed how limber he felt afterward.
Here are some examples of hair shirt as an adjective:
Carla felt so guilty about murdering her gardener that she chose to live a hair-shirt existence. She gave her belongings to charity and moved to the desert, where she survived by eating spiders and rats.
Getting healthy doesn’t mean living a hair-shirt lifestyle. Merv found vegetables to be delicious, and he got lots of dates from yoga class.