Her jaw was clenched.

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Mouth: Disagreement also shows up in compressed or pursed lips, clenched jaw muscles, or a head turned slightly away, so eye contact becomes sidelong.

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He showed a lack of energy (which made him seem detached), looked down too often, and his most-displayed expression when Romney was speaking was a lip compression with a clenched jaw.

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The presence of light-struck wash drawings of cloud-borne figures and angels by Giambattista Tiepolo proposes a reading of Degas's aerialist as a modern-day, secular version of such imagery, clearly suspended from her clenched jaw, not divinely supported.

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Typically, someone who is in agreement with you will smile and nod as you speak. (Disagreement shows up in compressed or pursed lips, clenched jaw muscles, or a head turned slightly away, so eye contact becomes sidelong.) But smiles are often used as a polite response and to cover up other emotions.

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