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.