being overheard; leaking out; being an open secret
Describes a situation where a conversation, plan, or secret is easily heard or known by others, often unintentionally. Commonly used in phrases like 筒抜けになる or 筒抜けだ.
Our conversation was completely overheard in the next room.
The company's internal affairs have become an open secret to rival firms.
going in one ear and out the other; not registering
Used when someone hears something but pays no attention or immediately forgets it, as if the information passes straight through without being absorbed. Often appears in the set phrase 聞いても筒抜け.
Even if I warn him, it goes in one ear and out the other.
Compound of 筒 (tube, pipe) and 抜け (passing through, coming out). The image is of sound or information passing straight through a tube without obstruction, giving rise to both the 'overheard/leaked' and 'in one ear and out the other' senses.