First recorded before 1000.
Comes from the Middle English word stedefast, from Old English stedefæst.
A combination of stead, "the place of a person," and fast, "firmly fixed in place."
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EXAMPLES OF STEADFAST

Despite facing numerous challenges, she remained steadfast in her commitment to finishing the marathon.
The community was grateful for the steadfast support of volunteers during the crisis.

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Did you mean invulnerable?