Most of these definitions have been disputed by various sources, they should be treated a source of entertainment only, not reference.
In the 1400's a law was set forth that a man was not allowed to beat his wife with a stick no thicker than his thumb. Hence we have "the rule of thumb"
In English pubs, ale is ordered by pints and quarts. So in old England, when customers got unruly, the bartender would yell at them "Mind your pints and quarts, and settle down." It’s where we get the phrase "mind your P’s and Q’s".
Baths consisted of a big tub filled with hot water. The man of the house had the privilege of the nice clean water, then all the other sons and men, then the women and finally the children, last of all the babies. By then the water was so dirty you could actually lose someone in it. Hence the saying, "Don't throw the baby out with the bath water."
In Greek mythology Thetis dipped her son in the mythical River Styx. Anyone who was immersed in the river became invulnerable. However, Thetis held Achilles by his heel. Since her hand covered this part of his body the water did not touch it and so it remained vulnerable. Achilles was eventually killed when an arrow hit his heel. Hence the saying, "Achilles heel".
Muskets had a priming pan, which was filled with gunpowder. When flint hit steel it ignited the powder in the pan, which in turn ignited the main charge of gunpowder and fired the musket ball. However, sometimes the powder in the pan failed to light the main charge. In that case you had a flash in the pan which didn't last long and didn't fire the musket ball. Hence the expression, "a flash in the pan".
In days gone by workmen carried their tools in sacks. If your employer gave you the sack it was time to collect your tools and go. Hence the expression, "to get the sack".
In the 16th century and the early 17th century if you went on a journey you could hire a horse to take you from one town to another and travel using a relay of horses. (That was better than wearing out your own horse on a long journey over very poor roads). In the early 1600s Thomas Hobson was a man in Cambridge who hired out horses. However, he would not let customers choose which horse they wanted to ride. Instead they had to ride whichever horse was nearest the stable entrance. So if you hired a horse from him you were given no choice at all. Hence the expression "Hobson's choice".
Rope made from hemp had a limited lifetime. When it wore out it was picked apart and recycled. It was used for caulking. Rope fibres (known as oakum) were hammered into the seams between planks of a ship and hot pitch was poured over it. This was done to waterproof the ship. Of course you got money for the old rope. So getting money for anything (seemingly) worthless was known as "money for old rope".