The degree to which a person's pants filter out his/her flatulence. The higher the fecal filter the less fart you smell.
Customer: Excuse me sir, could you tell me which of these brands of jeans has the highest fecal filter?
Salesperson: Of course. (Brand A) will give you the best protection. Wear (Brand B), however, and you might as well go commando; you can practically SEE the gas wafting out.