johnson · noun1
Compare the use of other men’s names as slang terms for the penis, e.g. peter and others listed in that entry.
Johnson · proper noun, noun2
A patronymic surname from John + -son, literally “son of John”. * In the U.S. the name Johnson has been adopted by numerous European immigrant families originally named Johansson, Johnsson, Johansen, Jonsson, etc. * Doublet of Johansson, Johansen, Jansen, and Hansen.
Etymology from Wiktionary, CC BY-SA 4.0.