Anything that's very fast or quick in terms of speed.
Example: The cheetah is the fastest land animal, capable of reaching speeds up to 70 mph. Some would describe that incredible agility by saying the animal is as quick as "greased lightning."
Grease is well known for being used to make things work better and faster. Lightning is well known for its speed. So what happens when you apply grease to a lightning bolt? Well, you get even faster lightning bolt. Yep, that's the idea behind the phrase.
The expression was in use during the 1800s, but it possibly goes back even further. One of earliest uses of the phrase was by recorded in an English newspaper, The Boston, Lincoln, Louth & Spalding Herald, on January 1833:
"He spoke as quick as 'greased lightning.'"
Reference: (1) ucar.edu - Lightning Fact Sheet, (2) newspaperarchive - contained digital copy of the newspaper with the expression.
Note: The definitions for sayings can be found with little effort, but finding the phrase origins is far more challenging to do. Taking a look back in history, it's strenuous to find the person or even the place where a phrase originates from. We are limited only to the things that can be found in writings. These include books, poems, newspapers, or plays. Often times, phrases will be quoted from century old newspapers, or from plays that were done in the 17th century by playwrights like William Shakespeare.
The sayings that are quoted are likely already commonly known, and have their origins elsewhere. For instance, if an idiom is printed in a newspaper from the 1820s, does that mean the idiom originated from that newspaper? No, it doesn't, but what it does tell you is that the phrase has been in use since the 1820s, so its origins are, in the least, over 200 years old.
knowyourphrase.com - Meanings and Origins of Phrases and Sayings!
* One of my roomates had an essay due in a couple days, and you could tell because his fingers were moving across the keyboard as fast as greased lightning!
* This man speaks as quick as greased lightning; I can't even make out anything that's he's saying... someone tell him to slow down.
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