On April 17, 1912 the New York Herald ran the headline "President Moves to Stop Mob Rule of Wireless." Then President William Taft had taken an interest in regulating the wireless. (It was an election year) Four months later congress passed the "Act to Regulate Radio Communication." It created a system to assign call letters and allocated frequencies. By creating legitimate, licensed radio, by default they also created pirate radio.Meaning "one who takes another's work without permission" first recorded 1701; sense of "unlicensed radio broadcaster" is from 1913. The verb is first recorded 1570s. From the Greek peirates, Latin Pirata meaning brigand.
Despite the 1912 date, I found a no references contemporary to that era. Pirates were still swashbucklers and not broadcasters in that decades. It is easy to find references from after 1940, very had to find them before 1930. As always it's difficult to ascertain the absolute first usage, change is always more cumulative in terms of changing language. Below I list several notable references. You can see as they move back in time the words pirate and radio are still used in context, but not in conjunction. The illegality becomes about the patent violation or use of the use of the frequency allocation. In other words, it becomes a verb.But from the quotes used in that 1935 reference you can see it's usage was still tenuous."
1936 - Popular Mechanics (January issue)
"Without knowing it, you probably have encountered the invisible "pirates" of the airwaves—those racketeers of space who today constitute a threat to national and international broadcasting of news and information."
1935 - Intermedia: Volume 13
"Their appreciation of music is clearly that which has been labeled 'young' over the years (funk, soul, rock, disco, etc); they have no significant preference for pirate radio stations as compared to others."
1934 - America: A Catholic Review of the Week
"In Havana however anti-American feelings have not subsided. On the heels of a broadcast from a pirate radio station urging action against American financiers and their property, the Woolworth store was bombed on October 17 with damage estimated at $2500. "
1934 - Modern Mechanix, (March Issue)
"All the so-called “radio pirates” are not across the border or out on the high seas. A. D. Ring, principal engineer of the-federal radio commission, says that at least 200 outlaw stations have been under surveillance in the United States alone."
1933 - Time magazine, Volume 22, Issue 9
"...Last week the Oppositionists, though their leaders still maintained contact with Mediator Welles, broadcast throughout Cuba from a pirate radio station..."
1931 - Popular Mechanics (August Issue)
"Monitoring of stations operating on the frequencies which can be measured at Grand Island cover the following classes of service transoceanic, telegraph and telephone, marine, broadcasting amateur, television and facsimile—to say nothing of the pirate stations among which is to be numbered the bootleg station"
1927 - Radio Broadcast Vol. 11
"...at the moment, the very foundations of broadcasting are threatened by the effort of a few inferior pirate stations to set aside the Radio Act..."
1926 - Wireless World: Volume 19
"Innumerable pirate builders of radio sets belong to these societies who must damage the legitimate radio manufacturer..."
1926 - The Outlook, Volume 143
"Secretary Hoover has expressed the opinion that National radio conferences are at an end until Congress passes legislation. In the meantime if stations continue to "pirate" wave-lengths broadcasting will stir the ether..."
1926 - The Century Magazine (October Issue)
"Until Congress acts this winter the temptation to pirate some other station's time or wave-length will always be present..."
1918 - House Committee on Merchant Marine...
"Very early—as early as 1904—the Navy Department began equipping naval vessels with piratical apparatus purchased from companies that made apparatus and sold it embodying the Marconi patents. The Marconi Co. protested to the Navy Department against that practice, and said it was not fair to a patentee to have the United States Government, which granted the patent, accept goods and buy them from a pirate."