If something is mossy, it is covered with moss, or it looks and feels like moss: soft, green, and a bit damp. A mossy rock, a mossy tree trunk, a mossy path. The word describes a place or a thing; it is not the name of the plant itself. It is an ordinary describing word, not slang, and it often sounds a little countryside-like or story-like. Though actually, people sometimes say mossy just to mean old, unused, or left alone too long, because moss grows where things sit still and stay wet.