means A pasty mixture of lime or gypsum, sand, and water that's spread wet and dries hard — used to coat walls and ceilings, mold sculptures, or set broken bones.

from From Greek 'emplastron,' something daubed or molded on, via Latin 'emplastrum' (a healing salve or dressing). The word arrived in English meaning a medicinal patch slapped on a wound — which is why a sticking bandage is still a 'plaster' in British English. The building-material sense, the wet stuff troweled onto walls, came along the same root: both are things you smear on and let set.