In modern times, this often refers to the odious practice (seen everywhere from IRC to supposedly respectable newspapers) of leaving in "indecent" words, but partially obscuring them: for example, "I have to take a sh*t" or "f— your mother!". It adds obscurity without reducing vulgarity, resulting in the worst of both worlds. As lj points out, it also draws the eye directly to the mangled word, which looks out of place.