knothole; peephole
Literal hole in wood where a knot has fallen out, or a small hole for looking through.
Light was leaking through a knothole in the old floorboard.
He peeked into the room through a peephole in the door.
poor eyesight; tunnel vision
Figurative use, often in set phrases like 節穴か or 節穴もいいところ, implying someone fails to notice obvious things.
It's right here and you didn't notice? Are your eyes just for show?
That referee is blind. He missed an obvious foul.
Standard kanji spelling for both literal and figurative senses.
Search-only form; not a standard written variant.
Compound of 節 (ふし, 'knot in wood') and 穴 (あな, 'hole'). The figurative sense comes from the idea that a knothole is a useless hole, implying eyes that do not function properly.