Shake (unit)
Informal unit of time equal to 10 nanoseconds
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Through Time, Second and Nuclear physics
Etymology
Like many informal units having to do with nuclear physics, it arose from top secret operations of the Manhattan Project during World War II. The word "shake" was taken from the idiomatic expression "in two shakes of a lamb's tail", which indicates a very short time interval.
The phrase "a couple of shakes", in reference to the measurement of time, may have been popularized by Richard Barham's Ingoldsby Legends (1840); however, the phrase was already part of vernacular language long before that.
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Nuclear physics
For nuclear-bomb designers, the term was a convenient name for the short interval, rounded to 10 nanoseconds, which was frequently seen in their measurements and calculations: The typical time required for one step in a chain reaction (i.e. the typical time for each neutron to cause a fission event, which releases more neutrons) is of the order of 1 shake, and a chain reaction is typically complete by 50 to 100 shakes.