Lambs are playful creatures and move remarkably fast. Hence, “two shakes of a lamb’s tail” means “very quickly” and has done so since at least 1840 in America. (Cool Cats, Top Dogs, and Other Beastly Expressions, Christine Ammer, 1999)
Where the phrase come from? We don’t know, but people generally agree that lambs can shake their tails so fast that two shakes is almost as fast as one. We seem to use it to refer to something we’re doing ourselves, for I’ve never heard anyone ask for something to be done in two shakes of a lamb’s tail.
The phrase first appeared in Ingoldsby Legends by Richard Barham, published in 1840. But it was used with such ease in a news article in 1853 that we can assume the phrase existed in modern language long before 1840.
Here is the first part of a letter to the editor which appeared in the Nelson Evening Mail in New Zealand in 1881: “A Brooklyn man spent seven hours writing an essay to prove that a woman is inferior to a man, and then spent two hours more and a heap of profanity in an ineffectual attempt to thread a needle, a job which a woman finally did for him in about two shakes of a lamb’s tail.”
The phrase is also used in science. This surprised me, because it seems like a very unscientific way of measuring something. But keep reading!
A shake is an informal unit of time equal to 10 nanoseconds. It has applications in nuclear physics, helping to conveniently express the timing of various events in a nuclear explosion. The typical time required for one step in the chain reaction (i.e. the typical time for each neutron to cause a fission event which releases more neutrons) is of order 1 shake, and the chain reaction is typically complete by 50 to 100 shakes.
This is also applicable to circuits. Since signal progression in IC chips is very rapid, on the order of nanoseconds, a shake is good measure of how quickly a signal can progress through an IC.
Like many nuclear units, it is derived from Top Secret operations of the Manhattan Project during World War II. The word comes from the expression “two shakes of a lamb’s tail,” which indicates a very short time interval. For nuclear-bomb designers, 10 nanoseconds was a convenient specific interval to connect to this term.
I’m still surprised. Lambs simply don’t fit with nuclear explosions, or vice versa. But it does prove that people can be creative with the language!