I've always found phrase origins fascinating - they can give us a glimpse into an entirely different reality, and, perhaps more importantly, they remind us of how many things we accept without question. We should definitely be questioning more.
Some examples of origins you probably don't know:
"Hang up the phone."
This comes from one specific kind of land-line phone that had a kind of hook you'd hang the handset from when you were done. Doing so would pull down the hook that was connected to a switch inside the phone that would disconnect the line.
Originating from printing houses in the days of moveable type:
Upper Case and Lower Case:
The individual type blocks used in hand typesetting are stored in shallow wooden or metal drawers known as "type cases". Each is subdivided into a number of compartments ("boxes") for the storage of different individual letters.
The terms upper and lower case originate from this division. By convention, when the two cases were taken out of the storage rack, and placed on a rack on the compositor's desk, the case containing the capitals and small capitals stood at a steeper angle at the back of the desk, with the case for the small letters, punctuation and spaces being more easily reached at a shallower angle below it to the front of the desk, hence upper and lower case.
"Mind your Ps and Qs":
A warning to printers’ apprentices to take care when selecting the little blocks with letters on, to ensure that they didn’t confuse the P block with the Q block, and vice versa.
Cut (or copy) and paste:
The term "cut and paste" comes from the traditional practice in manuscript-editings whereby people would cut paragraphs from a page with scissors and paste them onto another page. This practice remained standard into the 1980s.
To run through the wringer (or put through the wringer) referred to this laundry device that predated the spin cycle.
There's lots of ship-related stuff (more here):
Above Board - Anything on or above the open deck. If something is open and in plain view, it is above board.
As the Crow Flies - When lost or unsure of their position in coastal waters, ships would release a caged crow. The crow would fly straight towards the nearest land thus giving the vessel some sort of a navigational fix. The tallest lookout platform on a ship came to be know as the crow's nest.
Between the Devil and the Deep Blue Sea - The devil seam was the curved seam in the deck planking closest to the side of the ship and next to the scupper gutters. If a sailor slipped on the deck, he could find himself between the devil and the deep blue sea.
Booby Hatch - Aboard ship, a booby hatch is a sliding cover or hatch that must be pushed away to allow access or passage.
Buoyed Up - Using a buoy to raise the bight of an anchor cable to prevent it from chafing on a rough bottom.
By and Large - Currently means in all cases or in any case. From the nautical: by meaning into the wind and large meaning with the wind: as in, "By and Large the ship handled very well."
Groggy - In 1740, British Admiral Vernon (whose nickname was "Old Grogram" for the cloak of grogram which he wore) ordered that the sailors' daily ration of rum be diluted with water. The men called the mixture "grog". A sailor who drank too much grog was "groggy".
Leeway - The weather side of a ship is the side from which the wind is blowing. The Lee side is the side of the ship sheltered from the wind. A lee shore is a shore that is downwind of a ship. If a ship does not have enough "leeway" it is in danger of being driven onto the shore.
Pipe Down - Means stop talking and be quiet. The Pipe Down was the last signal from the Bosun's pipe each day which meant "lights out" and "silence".
Pooped - The poop is the stern section of a ship. To be pooped is to be swamped by a high, following sea.
Rummage Sale - From the French "arrimage" meaning ship's cargo. Damaged cargo was sold at a rummage sale.
Scuttlebutt - A butt was a barrel. Scuttle meant to chop a hole in something. The scuttlebutt was a water barrel with a hole cut into it so that sailors could reach in and dip out drinking water. The scuttlebutt was the place where the ship's gossip was exchanged.
Skyscraper - A small triangular sail set above the skysail in order to maximize effect in a light wind.
Slush Fund - A slushy slurry of fat was obtained by boiling or scraping the empty salted meat storage barrels. This stuff called "slush" was often sold ashore by the ship's cook for the benefit of himself or the crew. The money so derived became known as a slush fund.
The Bitter End - The end of an anchor cable is fastened to the bitts at the ship's bow. If all of the anchor cable has been payed out you have come to the bitter end.
The Devil to Pay - To pay the deck seams meant to seal them with tar. The devil seam was the most difficult to pay because it was curved and intersected with the straight deck planking. Some sources define the "devil" as the below-the-waterline-seam between the keel and the the adjoining planking. Paying the Devil was considered to be a most difficult and unpleasant task.
Three Sheets to the Wind - A sheet is a rope line which controls the tension on the downwind side of a square sail. If, on a three masted fully rigged ship, the sheets of the three lower course sails are loose, the sails will flap and flutter and are said to be "in the wind". A ship in this condition would stagger and wander aimlessly downwind.
To Know the Ropes - There was miles and miles of cordage in the rigging of a square rigged ship. The only way of keeping track of and knowing the function of all of these lines was to know where they were located. It took an experienced seaman to know the ropes.
And, moving beyond the nautical:
Post in the sense of "a blog post": from the act of nailing a sign to a wooden post, or from sending a message via a system of posted riders and horses (i.e., they are assigned to their stations, a.k.a. their posts).
Run of the Mill: Mills have very large and heavy grinding stones, it takes some time to get to normal speed for grinding grains. At the operational speed, they produce a nice, fine and uniform powder or grounded grains (flour etc.), this "steady state" output was called run of the mill. Which has come to mean "ordinary".
Software bugs and debugging: In the context of computers it uses was popularized by Grace Murray Hopper, involving an electromechanical fault in the Mark II computer, due to a moth stuck in one of the relays.
Know of any others? Leave them in the comments!