The Collaborative International Dictionary of English v.0.48:

Connascence \Con*nas"cence\, Connascency \Con*nas"cen*cy\, n.
   [L. con- + nascentia birth, fr. nascens, p. pr. of nasci to
   be born.]
   1. The common birth of two or more at the same tome;
      production of two or more together. --Johnson.
      [1913 Webster]

   2. That which is born or produced with another.
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   3. The act of growing together. [Obs.] --Wiseman.
      [1913 Webster]