The Collaborative International Dictionary of English v.0.48:

Familiar \Fa*mil`iar\, a. [OE. familer, familier, F. familier,
   fr. L. familiaris, fr. familia family. See Family.]
   1. Of or pertaining to a family; domestic. "Familiar feuds."
      --Byron.

   Syn: familial.
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   2. Closely acquainted or intimate, as a friend or companion;
      well versed in, as any subject of study; as, familiar with
      the Scriptures.
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   3. Characterized by, or exhibiting, the manner of an intimate
      friend; not formal; unconstrained; easy; accessible. "In
      loose, familiar strains." --Addison.
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            Be thou familiar, but by no means vulgar. --Shak.
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   4. Well known; well understood; common; frequent; as, a
      familiar illustration.
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            That war, or peace, or both at once, may be
            As things acquainted and familiar to us. --Shak.
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            There is nothing more familiar than this. --Locke.
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   5. Improperly acquainted; wrongly intimate. --Camden.
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   Familiar spirit, a demon or evil spirit supposed to attend
      at call. --1 Sam. xxviii. 3, 7-9.
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WordNet (r) 3.0 (2006):

familiar spirit
    n 1: a spirit (usually in animal form) that acts as an assistant
         to a witch or wizard [syn: familiar, familiar spirit]

Easton's 1897 Bible Dictionary:

Familiar spirit
   Sorcerers or necormancers, who professed to call up the dead to
   answer questions, were said to have a "familiar spirit" (Deut.
   18:11; 2 Kings 21:6; 2 Chr. 33:6; Lev. 19:31; 20:6; Isa. 8:19;
   29:4). Such a person was called by the Hebrews an _'ob_, which
   properly means a leathern bottle; for sorcerers were regarded as
   vessels containing the inspiring demon. This Hebrew word was
   equivalent to the pytho of the Greeks, and was used to denote
   both the person and the spirit which possessed him (Lev. 20:27;
   1 Sam. 28:8; comp. Acts 16:16). The word "familiar" is from the
   Latin familiaris, meaning a "household servant," and was
   intended to express the idea that sorcerers had spirits as their
   servants ready to obey their commands.