no fear

The opposing senses of the interjection derive from the phrase functioning as an ellipsis of both have no fear ("be assured") and there is no fear of that happening ("it's impossible").

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no fear

1. Not a chance, impossible.
   • 1834, L[etitia] E[lizabeth] L[andon], chapter XII, in Francesca Carrara. […], volume II, London: Richard Bentley, […], (successor to Henry Colburn), →OCLC, page 139:

     No fear, therefore, that, suddenly deprived of companionship, he should feel dull, and that dulness might take the shape of remorse; so repent, return, and be forgiven.

no fear

1. Synonym of never fear: no worries, don't worry.

   You want me to handle it? No fear.

   • 1918, Mary Austin, The Trail Book:

     "No fear," he said. "Every man will know his own when I come to the Council."

2. (British, dated) Synonym of no way: absolutely not.

   You're afraid I would handle it? No fear.

   • 1904, M. R. James, Ghost Stories of an Antiquary:

     "Why, did you show him up some rot?"
     "No fear," he said.

   • 1947, Dorothy L. Sayers, Creed or Chaos:

     Q: Will thou be baptized in this faith?
     A: No fear!

   • 1992, The Animals of Farthing Wood, episode 2:

     Toad: You should take up swimming!
     Rabbit: No fear, toad!

The two senses of the phrase are more or less antonymic: one can express a desire for something to proceed, while the other usually expresses a desire for something not to proceed. Since the phrase is a contranym, care should be taken to avoid ambiguity or confusion.

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