Definitions for knotholeˈnɒtˌhoʊl
Random House Webster's College Dictionary
a hole in a board or plank formed by the falling out of a knot or a portion of a knot.
Origin of knothole:
a hole in a board where a knot came out
In a piece of lumber, a knothole is a void left by a knot in the wood; such holes are often convenient for peering through when they occur in fences.
Origin: knot + hole