A Jiffy Is an Actual Unit of Time

When someone says 'I'll be back in a jiffy,' they're using a real scientific unit of time. In computing, one jiffy equals 1/100th of a second (10 milliseconds). In physics and chemistry, a jiffy is the time it takes light to travel one centimeter — approximately 3.3×10⁻¹¹ seconds.

The term was first used scientifically in 1867 by Gilbert Newton Lewis in chemistry. Today it appears in operating system timing and electrical engineering. So technically, being back 'in a jiffy' means you need to return in about 10 milliseconds!