The Collaborative International Dictionary of English v.0.48:

Atonic \A*ton"ic\, n.
   1. (Gram.) A word that has no accent.
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   2. An element of speech entirely destitute of vocality, or
      produced by the breath alone; a nonvocal or surd
      consonant; a breathing. --Rush.
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   3. (Med.) A remedy capable of allaying organic excitement or
      irritation. --Dunglison.
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The Collaborative International Dictionary of English v.0.48:

Atones \At*ones\, adv. [See At one.] [Obs.]
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         Down he fell atones as a stone.          --Chaucer.
   [1913 Webster] Atonic \A*ton"ic\, a. [Cf. F. atonique. See
   Atony.]
   1. (Med.) Characterized by atony, or lack of vital energy;
      as, an atonic disease.
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   2. (Gram.) Unaccented; as, an atonic syllable.
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   3. Destitute of tone vocality; surd. --Rush.
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WordNet (r) 3.0 (2006):

atonic
    adj 1: characterized by a lack of tonus
    2: used of syllables; "an atonic syllable carries no stress"
       [syn: atonic, unaccented] [ant: accented, tonic]