The Collaborative International Dictionary of English v.0.48:

Baseborn \Base"born`\, a.
   1. Born out of wedlock. --Gay.
      [1913 Webster]

   2. Born of low parentage.
      [1913 Webster]

   3. Vile; mean. "Thy baseborn heart." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

baseborn
    adj 1: of low birth or station (`base' is archaic in this
           sense); "baseborn wretches with dirty faces"; "of humble
           (or lowly) birth" [syn: base, baseborn, humble,
           lowly]
    2: illegitimate [syn: base, baseborn]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "baseborn":
   base, bastard, below the salt, cockney, common, commonplace, false,
   fatherless, homely, humble, illegitimate, low, lowborn, lowbred,
   lowly, mean, misbegotten, miscreated, natural, nonclerical,
   ordinary, plain, plebeian, rude, shabby-genteel, spurious,
   supposititious, third-estate, ungenteel, unwashed, vulgar