The Collaborative International Dictionary of English v.0.48:

Unbosom \Un*bos"om\, v. t. [imp. & p. p. Unbosomed; p. pr. &
   vb. n. Unbosoming.] [1st pref. un- + bosom.]
   To disclose freely; to reveal in confidence, as secrets; to
   confess; -- often used reflexively; as, to unbosom one's
   self. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unbosom
    v 1: relieve oneself of troubling information [syn: unbosom,
         relieve]