one who continuously comes back for more punishment. not a masochist per se, as they are usually after straight up punishment. a glutton for punishment isn't only punished, they are usually rewarded in some way as well.

example:
i routinely shop at the Target of denial when i desire mediocre consumer-grade goods. i usually never get everything i want in one shopping trip - there's always leftovers for my next shopping trip. i'm a glutton for punishment as i choose to shop there instead of driving all the way out to the Wal*Mart Where People Beat Their Kids, even though i'll be able to get everything i want in one trip if i go there.