the.com/knot hole

means A hole left in a board or trunk where a branch knot used to be, often formed as the knot dries out and falls away or is knocked loose.

from Straightforward compound: 'knot,' from Old English 'cnotta' (a hard lump, originally the tangled binding sense we still use for rope), applied early on to the dense whorled wood where a branch joins a trunk; plus 'hole,' from Old English 'hol.' When lumber is cut and dries, that harder knot tissue shrinks and separates from the surrounding wood faster, and can pop out — leaving the gap carpenters and fence-builders have been cursing and peering through for centuries.