From Daryl Stout@HURRICAN/THUNDER to All on Sat Mar 4 10:14:00 2017
The telegraph call CQ was born on the English Telegraph nearly a century
ago as a signal meaning "All stations. A notification to all postal
telegraph offices to receive the message." Its meaning was close to the
present meanings of QNC and QST. Like many other telegraph terms which originated on the landlines, CQ was brought over into radio and used as
a general call to all ships by the Marconi Company. Other companies used
KA until the London Convention of 1912, which adopted CQ as the
international general call or "attention" signal. CQ still means,
literally, "attention" but in amateur radio its meaning is perhaps more accurately described by Thomas Raddell who compared it to yelling "Hey,
Mac!" down a drain pipe.
But why the letters CQ? It's apparently from the French word for safety...
or, as intended here, pay attention.