The Collaborative International Dictionary of English v.0.48:

Unbid \Un*bid"\ ([u^]n*b[i^]d"), Unbidden \Un*bid"den\, a.
   1. Not bidden; not commanded.
      [1913 Webster]

            Thorns also and thistles it shall bring thee forth
            Unbid; and thou shalt eat the herb of the field.
                                                  --Milton.
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   2. Uninvited; as, unbidden guests. --Shak.
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   3. Being without a prayer. [Obs.] --Spenser.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "unbidden":
   arbitrary, autonomous, discretional, discretionary, elective, free,
   free will, gratuitous, independent, nonmandatory, offered,
   optional, proffered, self-acting, self-active, self-determined,
   self-determining, spontaneous, unacceptable, unagreeable, unasked,
   unasked-for, unbesought, uncalled-for, uncoerced, uncompelled,
   undesirable, undesired, unforced, uninfluenced, uninvited,
   unpressured, unprompted, unrequested, unrequired, unsolicited,
   unsought, unwanted, unwelcome, unwished, voluntary, volunteer,
   willful