松 (sōng): To loosen, relax, or slacken. Think of loosening a tight belt or relaxing your tense shoulders.
了 (le): A grammatical particle indicating a completed action or a change of state. It shows that the “loosening” has happened.
一 (yī): One.
口 (kǒu): Mouth. Here it acts as a measure word for a “mouthful” or a single action done with the mouth.
气 (qì): Breath, air, or vapor.
When combined, “松了一口气” literally translates to “loosened one mouthful of breath.” This paints a perfect picture of the physical act of exhaling deeply after holding your breath in anticipation or stress, a universal sign of relief.