v. broad·cast or broad·cast·ed, broad·cast·ing, broad·casts
1. To transmit a radio or television program
2. To send out or communicate, especially by radio or television.
3. To make known over a wide area: broadcast rumors.
4. To sow seed over a wide area, especially by hand.
The term originates in the 1700s as a reference to the spreading of seeds as a compound word that describes the actual throwing of seed broad + cast. It was literally the wide swing of the hand to spread seed over a large area, a wasteful technique used in the planing of grain. in fact modern mechanical contrivances that mimic this motion are called "broadcast seeders." The modern meaning can be definitively dated to 1921 according to most texts. But de Forest was using it in his journals as early as 1907. I've found uses in radio before that date as well. You can see some of them use it in a metaphorical way as it may have been outside the radio context, but it serves to show how the word transmuted over time. The clear change is when the writer uses broadcast as a verb. One instance even uses it twice, once in each sense.
1920 - Hearings before the Committee on the Merchant Marine and Fisheries... From the statement of Mr. S. M. Kintner of the International Radio Telegraph Co.
"... The press associations do not want their news handed to people who do not pay them for it, as expressed by the one that objected to our broadcasting the news delivered by them to one of their subscribers.
October 1920 - The Bulletin of the American Meteorological Society - Volume 1
"The caution was broadcast by wireless and placed vessel masters on the alert."
1918 - Report of the Commissioner of Education made to the Secretary of the Interior...
"During this period 36 messages of an average length of 1,500 words were broadcast, released twice each week, on Monday and Thursday evenings.
September 1918 - Popular Mechanics
"By broadcasting is meant the sending of messages so that any vessel, battleship, or merchantman, with a properly tuned instrument, can receive it."
1915 - Proceedings of the Institute of Radio Engineers, Vol. 3
"On the Pacific coast, time signals are sent broadcast to sea, thru the Naval stations at Mare Island, Eureka, San Diego, in California, and at North Head, Washington."
1915 - Wireless World, Vol. By Wireless Society of London
"At 6 p.m. (75th Meridian time) ice information will be sent broadcast for the benefit of vessels using 600 metre wave-length. .."
1913 - West Indies Pilot, Volume 1, By The United States Hydrographic Office
"Information concerning wrecks, derelicts, and other dangerous obstructions to navigation whenever received from the Hydrographic Office or from a branch Hydrographic Office is sent broadcast four times daily, viz, at 8 a.m., noon, 4 p.m., and 8 p.m., local (standard) time of station.
August 1913 - Report of the Hydrographer to the Bureau of Navigation
"...A great deal of this has been concerning dangers to navigation, which has been sent broadcast by the naval radio station at Arlington and by some of those on the coast. This method of broadcasting information concerning dangers to navigation is of great use to shipping and is greatly appreciated."
March 1909 - Popular Mechanics
"Sixty-five miles from shore the stricken steamer flashed electrical calls for help broadcast across the ocean."