"in a Jiffy"

To do something or get somewhere "in a jiffy", meaning to get there quickly or to do a task expediently. What is a "Jiffy?"
Thanks

Merriam-Webster doesn't have an origin. Some other reference might.

Main Entry: jif·fy
Pronunciation: 'ji-fE
Function: noun
Inflected Form(s): plural jiffies
Etymology: origin unknown
Date: 1779

MOMENT, INSTANT

The Oxford English Dictionary says "Origin unascertained". The earliest use it gives is from Munchhausen's Travels : "In six jiffies I found myself and all my retinue . . . at the rock of Gibralter [sic]". I'm told that the term was taken into the scientific community late in the 20th century and was actually applied as a unit of time; however the unit seems to vary from discipline to discipline!