Boyd’s Bible Dictionary:
(forming on). Used by Hebrews as wall and stone coating (Lev. 14:42; Deut. 27:2,4; Dan. 5:5).
Concise Bible Dictionary:
This was used to cover the walls of houses (Lev. 14:42-48; Dan. 5:5); and was also spread on large stones, on which the law could be inscribed (Deut. 27:2-4; Josh. 8:32). It may have been compounded of different substances for divers purposes. In Isaiah 38:21 plaster is used in a medical sense as spread on a boil.
Strong’s Dictionary of Hebrew Words:
Meaning:
a primitive root probably meaning to boil up (compare 7736); used only as denominative from 7875; to plaster