"Slacker": A lazy person, one who avoids work
Derivation: Lines on a ship work often in opposing pairs, like muscles. When one line is hauled on, the other must be loosed, or slacked. Obviously, the hauling was much harder work than slacking.
"Mainstay": A vital person, or a most important part of an organization
Derivation: A stay is a line, part of the standing rigging that supports a mast from the front or rear, to keep it from falling over backwards or forwards. The mainstay is the line that supports the main mast, the tallest mast on the ship.
"Loose Cannon": A person or thing that is uncontrolled and wildly destructive
Derivation: A ship's cannon was several tons of cast iron, mounted on a wheeled carriage. It was lashed very securely by heavy ropes to its place on the gun deck. It ever broke loose, especially on a rolling sea, it could smash all sorts of things, even poke holes in the side of the ship, or kill someone; and of course, being so heavy, it was very hard to rein in again. Definitely something to be avoided.
"Scuttlebutt": Rumors, gossip, informal talk
Derivation: A butt was a word for a small barrel. A scuttle was a drain hole. A scuttlebutt was a water barrel, around which sailors would gather and exchange gossip, much like the water cooler in a modern office.
"Leeway": margin for error, wiggle room, tolerance
Derivation: The lee side was the side away from the wind (the opposite of the windward side). As a ship moved forward in a cross-wind, it was blown sideways to some degree as well. In other words, it made leeway as well as headway. This had to be allowed for in navigational plotting.
"The Bitter End": The absolute end of something
Derivation: The term referred to the unattached end of a line, which really was rarely come across. The attached end was called the working end.
"Show The Ropes": Familiarize, introduce, show how to do a job
Derivation: A tall ship had literally hundreds of ropes (although sailors always called them "lines.") They weren't labeled (most sailors were illiterate) so they had to be memorized.
"Taken Aback": Startled, stopped dead in one's tracks, immobilized by surprise
Derivation: If a sudden wind shift brought the winds in front of the sails instead of behind them, literally blowing them backwards, it could bring the ship to a sudden, shuddering halt.
"Groggy": In a stupor, half-asleep, seemingly drunk
Derivation: Grog was rum diluted by water, drunk by sailors, with the expected effects. It was first invented by a British Navy Sir Edward Vernon, whose nickname was "Old Grog" after the grogram (grosgrain) cloak he wore, ironically as a means of cutting down sailors' alcohol intake.