I often heard the cannonball version, but never really bought into it. A more plausible etymology is that it is a corruption of "cold enough to freeze the brass balls off a monkey"; a 'monkey' in this sense is a type of steam engine
. Took this off the 'net:
"It is a little-known fact that "cold enough to freeze the balls off a brass monkey" comes from the early days of steam engines. This phrase originally referred to the state of a flyball governor in an unheated machine shop when the engine
was idle. Steam engines were commonly used to drive machinery used in repairing ships, and winters in the coastal towns of New England
, some of which boasted of several shipyards, could be extremely cold. A shop foreman would sometimes come to work in the morning and find that the brass flyballs had popped loose from their shafts overnight. Talking with his counterparts in other shipyards, he usually found that they had made the same observation in their shops. In that circumstance, it became the custom to say "It was cold enough last night to freeze the balls off a brass monkey" http://www.phrases.org.uk/bulletin_b...ages/1154.html
I've long had a fascination with etymology, but found it can throw you a lot of red herrings (another mystifying saying). I can't recall
the source of this article - I used it several years ago teaching the history
of naval-slang to some "snotters":
· "mind your p's and q's"
This expression, meaning "be very careful to behave correctly",
has been in use from the 17th century on. Theories include: an
admonishment to children learning
to write; an admonishment to
typesetters (who had to look at the letters reversed); an
admonishment to seamen not to soil their navy pea-jackets with
their tarred "queues" (pigtails); "mind your pints and quarts";
"mind your prices and quality"; "mind your pieds and queues"
(either feet and pigtails, or two dancing figures that had to be
accurately performed); the substitution of /p/ for "qu" /kw/ in the
speech of uneducated ancient Romans; or the confusion by students
both Latin and Ancient Greek of such cognates as _pente_
and _quintus_. And yes, we've heard the joke about the instruction
to new sextons: "Mind your keys and pews."
The most plausible explanation is the one given in the latest
edition of Collins English
Dictionary: an alteration of "Mind
your 'please's and 'thank you's".