to swear by

Swear \Swear\, v. i. [imp. {Swore}, formerly {Sware}; p. p.
   {Sworn}; p. pr. & vb. n. {Swearing}.] [OE. swerien, AS.
   swerian; akin to D. zweren, OS. swerian, OHG. swerien, G.
   schw["o]ren, Icel. sverja, Sw. sv[aum]rja, Dan. svaerge,
   Icel. & Sw. svara to answer, Dan. svare, Dan. & Sw. svar an
   answer, Goth. swaran to swear, and perhaps to E. swarm.
   [root]177. Cf. {Answer}.]
   1. To affirm or utter a solemn declaration, with an appeal to
      God for the truth of what is affirmed; to make a promise,
      threat, or resolve on oath; also, to affirm solemnly by
      some sacred object, or one regarded as sacred, as the
      Bible, the Koran, etc.
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            Ye shall swear by my name falsely.    --Lev. xix.
                                                  12.
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            I swear by all the Roman gods.        --Shak.
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   2. (Law) To give evidence on oath; as, to swear to the truth
      of a statement; he swore against the prisoner.
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   3. To make an appeal to God in an irreverant manner; to use
      the name of God or sacred things profanely; to call upon
      God in imprecation; to curse.
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            [I] swore little; diced not above seven times a
            week.                                 --Shak.
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   {To swear by}, to place great confidence in a person or
      thing; to trust implicitly as an authority. "I simply
      meant to ask if you are one of those who swear by Lord
      Verulam." --Miss Edgeworth.

   {To swear off}, to make a solemn vow, or a serious
      resolution, to abstain from something; as, to swear off
      smoking. [Slang]
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