How fast is too fast?

The word “X” has been in use since the 18th century. Although the origins of the word are unclear it is believed to have originated from criminal slang for something that was fast as lightening.

From there, it moved into popular usage to describe quick things. In the 19th century, Gilbert Newton Lewis–an American chemist was the first to apply the term “X” to technical endeavors. He used “X” to stand in for the time it took light to travel one centimeter (approximately 33.4 picoseconds).

After Lewis’s application, the term crept into usage in other scientific pursuits. In electronics an “X” is the time between alternating power cycles (1/60th of a second for most systems). In computing, an "X" is one tick of the system timer (as such, a computing “X” is not a fixed unit of time but an OS/hardware dependent unit of time). In astro and quantum physics, an "X" is the time it takes light to travel one fermi (the width of a nucleon) and amounts to 3 × 10^-24 seconds

Although “X” has established itself in multiple scientific communities, it was hardly constrained to use by the technically inclined. Anytime you see an “X” Lube oil shop, “X” Pop popping corn you're seeing a 300 year old word alive and well in modern use.

ID X.