Dr. Samuel Johnson, Origin of a Penis Euphemism

In his forthcoming cultural history of euphemisms, Euphemania, Ralph Keyes offers the following (seemingly apocryphal) origin of the euphemism “Johnson” (for “penis,” of course) —

Johnson is the last name most often used for the male sex organ. According to one theory, this slangy euphemism originated with the name of a large railroad brake lever. Lexicographer Eric Partridge thought it was more likely an abbreviated version of Dr. Johnson, a onetime synonym for “penis” that Partridge said might be based on the assumption that ‘there was no one Dr. [Samuel] Johnson was not prepared to stand up to.’ Working under the verbal restraints of his times, Partridge said this synonym was for the ‘membrum virile.’