Find the word behind the meaning.

Her otherwise cheerful message carried an undertone of concern.

In a second discussion, the presenter made a caustic remark about the committee's planning skills, turning a legitimate concern into a cutting personal jab. His sardonic comment that another delay was exactly what the project needed conveyed bitter mockery rather than sincere approval. Beneath his later polite words, an undertone of resentment remained noticeable in the emphasis he placed on certain names.