Confidential Talk
Whispering
noun
Remember how 内緒 means "secret" or "confidential"? Well, that makes this is a secret talk, aka a confidential talk — like the kind often done by whispering.
This word uses the on'yomi of 内緒 and kun'yomi of 話, only rendakued.
当人達は内緒話をしているつもりのようだが、声がでかいので全部筒抜けだよ。
They are trying to whisper to each other, but their voices are so loud that I can hear everything.