Etymology- the study of the origin of words and the way in which their meanings have changed throughout history. 

*Note: Many of these origins are actually unknown and are considered folk etymology  

When “it’s raining cats and dogs”: 

Modern translation: When it is raining heavily   

Explanation 1: Mythology derivation where dogs and wolves were attendants to Odin, the god of storms. 

Explanation 2: In the 17th and 18th century, London’s heavy rains often washed the bodies of dead cats and dogs down the streets.  

Explanation 3: In the 16th century, thatched roofs—a roof made of layers of straw and other dry vegetation—were very common. In these roofs many animals such as dogs, cats, mice, bugs etc. were able to take refuge from the heavy rains “inside the roof”. The rains made straw slippery in which many animals would slip and fall off. 

To give someone “the cold shoulder”: 

Modern translation: Deliberately ignoring someone  

 Beginning in the 19th century, his phrase may have derived from when one was a guest at one’s house. If you were served a hot meal, you were welcome to stay. If served a cold meal, it was a hint to kindly leave. The shoulder specifically refers to a cold shoulder of mutton, which was often served.  

To be caught “red-handed”: 

Modern translation: Catch someone in the course of wrongdoing  

While there are many myths as to how the phrase became, it is actually very literal. 

Starting in the 15th century, originally just red-hand, it literally means having blood on one’s hand from murder or poaching. 

To “take with a grain of salt”: 

Modern translation: To accept something while also staying skeptical 

The overall idea comes from the fact that food is easier swallowed with “a pinch of salt”. Pliny the Elder translated an ancient antidote for poison with the words “be taken fasting, plus a grain of salt”.   

That was a “piece of cake”: 

Modern translation: An easily obtainable task  

In the 1870s, cake was handed out for competition prizes.  

In United States slavery states, salves would circle around a cake, in which the most graceful pair won the cake. It was also from this that “cake walk” derived from.  

To “bite the bullet”:  

Modern translation: To endure an unavoidable and unpleasant situation  

Explanation 1: Before proper anesthetics, it was said that soldiers should bite a bullet to help endure the pain. This was most likely derived from the time of the American Civil War.  

Explanation 2: Archaeologists have recovered musket balls with teeth marks, while this possibly gives this phrase it’s meaning, while the teeth-marked musket balls may also refer the fast-paced reloading on a battlefield where soldiers put the musket balls in their mouth for easier access.  

Sources: Oxford Dictionaries, https://en.oxforddictionaries.com/definition/etymology 

Origin of a Cliché, http://www.pages.drexel.edu/~zk32/clicheorigin.html 

The Phrase Finder, https://www.phrases.org.uk