noun
apologetic tone; defensive tone of voice
Describes a manner of speaking that sounds like one is making excuses or justifying oneself, often with a defensive or apologetic edge.
彼は遅刻の理由を弁明口調で話した。
He explained the reason for his lateness in an apologetic tone.
上司に注意されて、弁明口調で返事をしてしまった。
When my boss pointed it out, I ended up replying in a defensive tone.
Compound of 弁明 (explanation, justification) and 口調 (tone of voice).