From the O.E.D:
Pronunciation: Brit. /ˈdʒɒnsn/, U.S. /ˈdʒɑns(ə)n/
Origin: From a proper name. Etymon: proper name Johnson.
Etymology: < the common surname Johnson, probably as a humorous alteration of John n. (compare senses 4a and 6 at that entry)
1. Chiefly U.S. (common in African-American use). The penis. Also Jim Johnson. rare before late 20th cent.
1863 W. B. Cheadle Jrnl. 2 Feb. in Jrnl. Trip across Canada (1931) 108 Neck frozen. Face ditto; thighs ditto; Johnson ditto, & sphincter vesicae partially paralyzed.
1972 Screw 12 June 10/3 So I went to take my turn with the hopes of somehow getting my Jim Johnson wet.
1986 J. A. Friedman Tales from Times Square 94 One of the black guys was nearly demanding a warm-up, some contrivance to stiffen his Johnson before the main event.
2018 @GordonBibby 29 Aug. in twitter.com (O.E.D. Archive) I once had a friend tell me about his piercing, I asked where it was and without hesitation he whipped out his Johnson and showed me.
2. Chiefly British. A man who lives off a prostitute's earnings; a pimp. Now rare.
1954 R. Fabian London after Dark i. 54 Many men in London live upon the immoral earnings of prostitutes. The name for such men is ‘Ponces’ or ‘Johnsons’.
1960 ‘J. Ashford’ Counsel for Def. v. 66 You got Legs off... He was a Johnson.