"ZL3MH (later ZL2OQ and recently a silent key) brought the antenna to ham attention in 1949, giving credit to W5LHI and W0GZR for basic information on the design."
The key in this context of "SK" refers to a telegraph key. It is the primary tool and human-to-machine interface for hams. The telegraph key is the the ham as the paintbrush to the painter, the pen to the writer. Clearly the term must antedate the Morse key. The term could predate wireless, but could not predate land-line telegraph systems. Straight lever keys date back to the 1850s but large scale production was uncommon until around 1880.
But at the other end of the time line I see the term escaping the ham culture and appearing in contemporary books... both fiction and non-fiction. I'll list off a few more recent publications.
- 2011 - Survivors - James Rawles
- 2009 - Full Circle - Theodore Cohen
- 2009 - Is Anyone There? Communication? by Karim Buksh
- 2004 - Talk the Talk - Luc Reid
- 2004 - The Queen Off Broadway - Alexis Greene
- 2003 - Hello World - Danny Gregory
- 1987 - Unsinkable - Daniel Butler
But as we go backward into the 1970s the references disappear from pulp fiction and reappear in trade magazines and publications dedicated to amateur radio. It appears in the ARRL handbook of 1979. The of June 1975 issue of QST magazine lists W3NRE as a silent Key. They had been using the term since at least 1967. A 1969 issue of CQ magazine lists WA2QCQ as a silent key and multiple 1966 issues use the term as well.
The uses of the term I find before that only refer to an inactive or otherwise unused telegraph key. such as in this line from The Great Iron Ship by James Dugan. "In the beach hut the electricians sat by a dark index and a silent key. French telegraph shares slumped acutely on the Bourse and Exchange." But there is one lone use from the 1940s that clearly is in the same sense that CQ and QST use it. A 1943 issue of MSG Journal, the trade magazine of the Marine Department of the American Communications Assoc. The line reads "...don't flirt with high Voltages, that is, unless you want to get your name into MSG, under the sad notation "Silent Key".
The use in quotes is common in formal writing with new terminology making the jump from slang to common usage. So that's my best guess. It was likely new in the late 1930s, and popularized by QST and CQ magazine who used the term to precede obituaries. It's worth noting that by 1985 the term was fully entrenched. In 1985, Alan Tavor succeeded in establishing Israel's Silent Key Forest, a memorial to deceased hams.