Idioms

Fly off the Handle - This comes from the days before mass manufacturing. Poorly built axe heads would occasionally soar off of their handles, leading to dangerous and unpredictable results.
Steal Someone's Thunder - This idiom comes from the early 1700s, when an English playwright named John Dennis used a device to imitate the sound of thunder for one of his plays. The play was a flop, but other playwrights started using the device for their own plays to greater success.
Under the Weather - Modern speakers use this to refer to any kind of illness, for sailors it meant seasickness. The original phrase was "under the weather bow," referring to the side of the ship that took the brunt of bad weather. When storms made for choppy water, sailors would head below deck to brace themselves and try to stave off seasickness.
Ride Shotgun - English speakers use this to mean sitting in the passenger seat of a car. The phrase dates back to the early 1900s in the Wild West when the person sitting beside the driver in a coach would often carry a shotgun for protection.
Barking Up the Wrong Tree - In the early 1800s, dogs were commonly used for hunting. When a dog would identify prey that had run up a tree, the dog would bark at it furiously. When the prey jumped to a different tree, the dog would be left at the base of the original tree, confused, barking up at nothing.

Jump on the Bandwagon - The origin of this idiom dates back to the 1848 U.S. Presidential election when a famed circus owner supported the campaign of Whig Party candidate Zachary Taylor by inviting him on his horse-drawn carriage carrying a live orchestra during a parade to advertise his message and meet potential voters. Other members of Taylor's party realized what a good idea this was, and clamored to join him on the bandwagon during future parades.

Wordology, Idioms

Here are a few idioms that have preserved words that we no longer use by themselves. They are almost exclusively used in context, rather than stand-alone.

Eke is usually used as to 'eke out a living'. It comes from an old verb meaning to add, supplement, or grow. It is also the same word that gave us "eke-name" for additional name, which became "nickname."

Dint comes from the Old English where it originally referred to a blow struck with a sword or other weapon. It is now used as "by dint of something" where 'something' can stand for charisma, hard work, or anything you can use to accomplish something else.

Deserts, as in 'just deserts' comes from an Old French word for 'deserve', and it was used in English from the 13th century to mean that which is deserved. When you get your just deserts, you get your due.

Fro, as in 'to and fro' comes from the old English way of pronouncing from.

Kith, as in 'kith and kin' comes from an Old English word referring to knowledge or acquaintance. The expression "kith and kin" originally meant your country and your family, but later came to have the wider sense of friends and family.

Umbrage, as in 'take umbrage' comes from the French ombrage (shade, shadow), and it was once used to talk about shade from the sun. It took on various figurative meanings having to do with doubt and suspicion or the giving and taking of offense. To give umbrage was to offend someone, to "throw shade".

Shrift, as in 'short shrift' came from the practice of allowing a little time for the condemned to make a confession before being executed. In that context, shorter was never better.