The Collaborative International Dictionary of English v.0.48:

muckrake \muck"rake`\ (m[u^]k"r[=a]k`), v. i. [imp. & p. p.
   -raked; p. pr. & vb. n. -raking.]
   To seek for, expose, or charge, especially habitually,
   corruption, real or alleged, on the part of public men and
   corporations.

   Note: On April 14, 1906, President Roosevelt delivered a
         speech on "The Man with the Muck Rake," in which he
         deprecated sweeping and unjust charges of corruption
         against public men and corporations. The phrase was
         taken up by the press, and the verb to muckrake, in
         the above sense, and the noun muckraker, to designate
         one so engaged, were speedily coined and obtained wide
         currency. The original allusion was to a character in
         Bunyan's "Pilgrim's Progress" so intent on raking up
         muck that he could not see a celestial crown held above
         him.
         [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

muckrake
    v 1: explore and expose misconduct and scandals concerning
         public figures; "This reporter was well-known for his
         muckraking"