means The moist membrane lining the inner surfaces of organs and cavities that open to the outside world — like the nose, mouth, gut, and airways — which secretes protective mucus.

from Straight from Latin mucosa, the feminine form of mucosus, "slimy, mucous," itself from mucus, "slime, nasal discharge." That mucus traces back to a root meaning something like "to slip" or "slimy thing," likely a cousin of Greek myxa (mucus, and also the nozzle of a lamp). The full anatomical term is membrana mucosa, "mucous membrane" — medical Latin simply dropped the membrana and let the slime stand alone.