Green’s Dictionary of Slang

a student or amateur telegraphist, subseq. an amateur radio operator, i.e. one who makes a hobby of picking up and transmitting radio messages.

1919		in C.H. Darling Jargon Book.
1928		Collier’s 22 Sept. 26: The amateur radio ‘hams’ have the ends of the earth for neighbors [DA].
1931		Irwin Amer. Tramp and Und. Sl. 95: Ham. [...] an inexperienced telegraph operator.
1947		S. Lewis Kingsblood Royal (2001) 156: Were you ever a radio ham?
1950		K. Vonnegut ‘Thanasphere’ in Bagombo Snuff Box (1999) 23: Now hams all over the world would be building sets so they could listen in, too.
1960		E. De Roo Big Rumble 146: He picked the radio up and put it on his table. He had wanted to learn code and perhaps get a ham’s license some day.