The rejectamenta of plant-eating insects.
Frass refers both to the excrement of insects ("bug poop"), and the detritus of wood-boring insects (excavated pieces of wood that don't make it down the hatch). You might also find pieces of shed exoskeleton as part of the frass expelled from a tidy burrow or colony.
Frass around the home is evidence that you've got invertebrate company, such as cockroaches, bed bugs, termites or carpenter ants. Because of its small size, it can become airborne, inhaled, and contribute to health problems, such as asthma.
A few entrepreneurial companies catering to the organic gardener sell frass as a fertilizer/ nutritional supplement, as they might cow excrement, bird excrement, or worm excrement. (They cultivate the product by feeding vegetables, cactus and wheat bran to crickets).
Origin: from the German, fressen, to devour.
Debbie Hadley, "Everything You Ever Wanted to Know About Insect Poop," About.com, http://insects.about.com/od/insects101/a/bugpoop.htm (November 8, 2012)
Kim Hunt, "How to Identify Indoor Insects by Droppings," eHow.com, http://www.ehow.com/how_8592287_identify-indoor-insects-droppings.html (November 8, 2012)