The English language has many expressions that describe a fast-moving event. Some expressions relate to body movements. We say something happens in the blink of an eye or in a heartbeat. Other expressions are related to the world of nature.
For example, there is an expression related to velocity involving an animal. You might think the creature will be a big cat, such as a lion or maybe even a small but speedy animal, such as a hare（野兔）. But it is none of them. The expression today involves sheep-baby lambs to be exact.
Lambs are cute, but they are not really known for their lightning speed. Our expression is about a specific part of the animal—its tail. Lambs can shake their tails very fast. And when something happens quickly, we say it happens in "two shakes of a lamb's tail". Here is an example, "I'm going to the store. I'll be back in two shakes of a lamb's tail!" This expression is a cuter and more descriptive way of saying "quickly". Maybe instead of sending a text message with the letters BRB （be right back）, we could use the image of a lamb shaking its tail.
Being cute like a lamb may be fun, but it has its shortcomings. Cute things are often not taken seriously. And that is why Americans rarely use "two shakes of a lamb's tail" in a serious situation. For example, if a person gets hit by a car, people would say something like "Help is on the way!" Generally speaking, an emergency is not a good time to use cute animal expressions.