An item bought at stores like Bed Bath & Beyond. Its exact nature and purpose remains a mystery to single men. The only way to uncover its manifold secrets is to marry a woman, and then be forced to go shopping for one.
Friend: Hey! You wanna hang out?
Newly-wed man: Sorry, man. We have to go shopping for a duvet cover.
Friend: What the hell is that?
Newly-wed man: I dunno, but apparently we need one.