I can't remember a thread about sailing terms and traditions and definitions, etc., and people were starting to put things like this into another thread, so I thought I had better start a new one just for this purpose.
Here are a couple that were placed in another thread:
POSTED BY WOUTER:
How about "He is a loose cannon" ?
Comes from the old sailing battleships on which the mouth loaded cannons needed to be pulled back to relaod and than push forward out of their ports to fire. The cannons were on wheels therefor and kept in check by many ropes (ooops lines) and pully blocks. Sometime a cannon brok loose and the rolling of the ship in the waves would run the heavy cannon on wheels all around over the gundeck or just straight through the wood work on the otherside taking everything with it. Try to stop on of those ! Hence "loose cannon" = Big trouble
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POSTED BY JAKE:
Why male ear piercing of the left ear is cool and the right is not:
As I learned this morning from Tim Zimmermann's book "The Race", in the mid 1800's the world had just figured out that it made more sense to sail from Australia and China back to England (or to the eastern U.S.) by sailing the Southern Ocean in a route of circumnavigation. It became customary and dignified for sailors who had sailed around Cape Horn of South America to pierce the ear closest to the Cape as they blew past (and survived)...the left ear. And with this I presume that to pierce the right ear meant you were clearly headed in the wrong direction!
Mary,
Here's one I dropped in the female skipper thread. "Its kind of fun to look up common phrases to see how they derived. Its surprising how many come from nautical terms. For example, 'three sheets to the wind', which means someone out of control (usually drunk) comes from the fact that small sailing sloops have three sheets that control the sails; starboard and port jib sheets and a main sheet. If the sloop was 'three sheets to the wind', it meant that the boat had all its control lines flying downwind, not cleated to anything and therefore not controlling the sails or boat."
How about, "scuttlebutt" or "whole nine yards"?
David
H20
What about "Cold enough to freeze the balls off of a brass monkey". 
That one comes from when they used to store cannonballs on the deck, in a brass verticle treelike device that had arms extending out of it. The arms were curved so they could move a bit with the rolling of the ship, so they looked like monkey's arms. Naturally they were brass to minmize corrosion.
The balls actually were nestled into round rings on the end of the arms.
When it got really cold, the rings would shrink more than the balls so the pressure would build up and then something would trigger the release and the balls would pop out of the rings. "Cold enough to freeze the balls......" 
"Son of a gun" Refers to days of old when the men who used to operate the cannons on Naval ships were called "Guns".
Well when the ship would sail into town the crew would go out get some liquor and shall we say "local ladies" who might perchance, to visit the ship. Well the cannons were located in cubiholes on the sides of the ships, just big enough to squeeze into.... and nine months later.... a son of a "Gun" was born.
Now I would like to know the origins of "athwartship". I may have misspelled, but I see a word somewhat like this ocassionally. It's not in my copy of the "Oxford Dictionary".