means A thin, often whitish or grayish fungal growth that forms on damp surfaces — walls, fabric, leather, plants — leaving a musty smell and slow decay in its wake.

from From Old English 'meledēaw,' literally 'honeydew' — 'mele' (honey) plus 'dēaw' (dew). It first meant a sweet, sticky substance found on plants (what we'd now call honeydew, the residue of aphids), and the word's relatives stretch across the Germanic languages and possibly back to a root meaning 'honey.' Only later did the sense sour, drifting from sugary plant-dew to the moldy bloom of damp — so a word that began tasting of honey ended up smelling of the cellar.