From the 1969 ARRL "The Radio Amateurs Operating Manual"
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.
-Louise Ramsey Moreau WB6BBO/W3WRE