Dramatist John Dennis was a playwright, unsuccessful unfortunately. In 1704 he invented a way to have thunder sounds in play but because he wasn't a successful playwright someone else picked up the idea and used it themselves. (ahem, Macbeth) Dennis was overheard grumbling that his thunder had been stolen. ( source ) Today the phrase holds the same meaning as what Dennis was grumbling - when someone steals your thunder you are saying they stole your idea or the attention you were receiving. 

Back in the day (the western days) people were hired to ride shotgun to protect the driver of the stagecoach, most often the coaches carrying money and valuables. ( source ) The term has morphed into riding in the front passenger seat of a car. And as in all competitions there are rules for calling and riding shotgun as outlined here . 

The earliest citation of this phrase is found in the 1546 publication of "The Proverbs of John Heywood" which also has a number of other popular idioms as well. ( source ) The phrase is another way to ask, "What are you thinking about?" 

The origin of this phrase comes from the closed mouth sound of "mmm", made when someone is trying to keep their mouth shut. *grin*  ( source ) The phrase is used today to let someone know you'll keep your mouth shut and keep the secret. 

You can tell the age of a horse by how long his teeth are. That's the origin of this phrase, read about it here .  Today the phrase isn't so much about horses as it is just about old age. 

Sailing on a ship you need to know the ropes, which is the most likely origin for this common expression. Read about it here .  Today the phrase indicates needing to know all the methods involved for a task, to have a complete understanding of something. 

Although occasionally used before the Civil War, its widespread popularity can probably be traced to General Joseph Hooker, a Union soldier who was well-known for the liquor and whores in his camp. He was ultimately demoted, and Washington prostitutes were jokingly referred to as "Hooker's Division." ( source ) I'm pretty sure I don't need to explain what today's meaning of hookers is...and if you don't know, ask your parents - I'm not going to be the one to explain that to you! *wink* 

There are two possible theories for this expression.  One of them centers around the stock market crash of 1929. In American English, from 1805 phrase fly a kite "raise money by issuing commercial paper on nonexistent funds;" According to The Toronto Star -- April 4, 1993, Sunday, FINAL EDITION: The phrase - loosely meaning go do something futile - originated during the stock market crash of 1929. It referred to "the useless bits of paper being tossed out the window" after the crash, she explains. The other theory is attested by 1942, American English, probably tracing to the popular song of the same name (lyrics by Johnny Burke), sung by Bing Crosby in "The Star Maker" (1939): Go fly a kite and tie your troubles to the tail  They'll be blown away by a merry gale,  Go fly a kite and toss your worries to the wind  And they won't come back, they'll be too chagrined. (for sources click here and here ) Today it basicall

The phrase is found in flintlock muskets and their inability to fire correctly from time to time. Read about it here .  In modern times it means that something, or someone, had a lot of potential but never followed through. 

This expression, used to represent peace and reconciliation, has its origins in the book of Genesis in the Bible. You can read the account in Genesis here and more about the use of the phrase here . 

'Don't count your chickens' is one of the oldest, and possibly the wisest. The thought was recorded in print by Thomas Howell in New Sonnets and pretty Pamphlets, 1570: Counte not thy Chickens that vnhatched be, Waye wordes as winde, till thou finde certaintee Samuel Butler continued the pleasing rhyming in his expression of the proverbial advice, in the narrative poem Hudibras, 1664: To swallow gudgeons ere they're catch'd, And count their chickens ere they're hatched. ( source ) Using it today we mean that until something is final or actually happens as predicted we shouldn't count on it.

Being caught red-handed was a way to catch poachers back in the day, the red referring to the blood still on their hands from killing animals. ( source ) There seems to be some other ideas to the origin but this one seems to be the most plausible.   Today the phrase means being caught in the very act, for example the below picture of me when I was a wee one. 😏 When I was a wee one I got into my Grandma's powder puff and used it liberally on myself and covered the bathroom with the stuff. I was caught red-handed and a picture was taken because I was just too cute to scold! :) 

The meaning of this phrase is very literal as far as its origins go. It refers to when certain animals would trick pack dogs into thinking they were up a tree when in fact they had escaped. ( source ) Nowadays the expression is used to describe a person who is following the wrong lead, clue, trail. 

The origin of this phrase is found in the Bible. And its meaning is clear - a house cannot remain standing if it is divided - through the foundation especially. 

While defending a woman and her honor back in the day the knight would tie her colors on his arm as a show of his loyalty to her. It was a way to express emotions for her visibly.  ( source ) Shakespeare also gets some credit for the expression (not a big surprise is it?) in the play  Othello . ( source ) Today the phrase is used in a figurative way to communicate that emotions are visible and not hidden. 

There are a few theories out there about the origins of the expression "the real mccoy." Click here and read about the possibilities.  Regardless of where or who the expression originated with the meaning of it is clear - it's the authentic thing.  

This term has its origins in the Bible, the book of Leviticus to be exact. Read about the morphing of it here . 

Red herring was used to divert or distract bloodhounds from their search for fox. People who were against the sport would use it to mislead. ( source ) Today it is used to indicate a diversion of any kind.