noun (plural byssuses or byssi) Etymology: Middle English bissus, from Latin byssus, from Greek byssos flax, of Semitic origin; akin to Hebrew būṣ linen cloth 1. a fine probably linen cloth of ancient times 2. [New Latin, from Latin] a tuft of long tough filaments by which some bivalve mollusks (as mussels) adhere to a surface