e.g. I’ll be back in two shakes of a lamb’s tail.
Meaning: very quickly, in a very short time.
Origin: The expression means in no time at all because this is how quickly a lamb shakes their tale. It first appeared in Richard Harris Barham’s book Ingoldsby Legends published in 1840, but since then it has been reduced to two shakes (e.g. see you in two shakes). Probably the expression is gradually disappearing because fewer people grow up actually seeing lambs wag their tails. However, shake is a recognized time of unit, equal to 10 nanoseconds, so the phrase will probably be preserved among nuclear scientists.