gonyo-gonyo
Speaking so quietly and unclearly that the words are impossible to make out. Often done when someone is embarrassed or trying to avoid answering directly.
💡A student caught not doing homework — 'He just said gonyo-gonyo when the teacher asked.'
彼はごにょごにょ言って何を言っているか分からない。
He's muttering gonyo-gonyo and I can't understand a thing.
When asked about the missing report, he just mumbled ___.
報告書のことを聞かれて、彼は○○と言い訳をした。