Complained under one's breath

MUTTERED

Think of how someone might speak when they’re annoyed but trying not to be heard.

"Complained under one's breath" hints at speaking quietly in frustration or annoyance. The answer "MUTTERED" fits because muttering means talking in a low, indistinct voice, often when grumbling or complaining to yourself. There’s no wordplay here—it’s a straightforward definition where the clue describes the action directly.