The term 'say under one's breath' can be solved by two crossword solutions: UTTER (5 letters) and MUTTER (6 letters). Uttering is the act of speaking words out loud, while muttering is the act of speaking quietly or under one's breath. Both terms refer to speaking in a low voice, usually with little volume. This type of speaking is often used when someone is trying to express their thoughts without being heard by others.