c. 1600, "one who rakes muck," from muck (n.) + agent noun from rake (v.). Meaning "one who inquires into and publishes scandal and allegations of corruption among political and business leaders," popularized 1906 in speech by President Theodore Roosevelt, in reference to "man ... with a Muckrake in his hand" in Bunyan's "Pilgrim's Progress" (1684) who seeks worldly gain by raking filth.
The men with the muck-rakes are often indispensable to the well-being of society, but only if they know when to stop raking the muck. [T. Roosevelt, quoted in "Cincinnati Enquirer," April 15, 1906.]
Muckrake in sense "person who hunts scandal" is attested from 1872. To muckrake (v.) in the literal sense is from 1879; figuratively from 1910. Related: Muckraking.