I set my jaw - Tradução em português

One more question that came from "Divergent" book. When Tris says "I set my jaw."; what does it means?

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Hi, minha sugestão:

"Setting one's jaw" is an expression usually adopted to signify determination in a person. It might mean that here:
Ben is possibly acknowledging the enormity of the task (freeing his people from oppression and prejudice), but his face have a determined (and serious - grim) look.

From word reference:

:mrgreen: