underwear; briefs; panties

The Martha & The Muffins song off the _This Is The Ice Age_ album from the early 1980's entitled "Boy Without Filters" might have been the genesis of the term. Also one needs to include the image and function of cigarette filters and coffee filters.

What they're filtering is left up to the user: gas, sweaty ball odor, snatch stench, overpowering sexual passion, etc.