Becoming weak or unsteady, or speaking with hesitation and breaks in confidence.
From Middle English 'falteren', possibly from Old Norse 'faltrask' meaning to stumble. The word evolved to describe both physical stumbling and the nervous hesitation in speech that mirrors losing your footing.
Your voice actually falters the same way your legs do when you lose confidence—both involve the same nervous system shutdown, which is why public speakers sweat and their speech becomes choppy!
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