What is the origin of 'sleep tight'?

Read moreOne of the mysteries of the English language finally explained.

We use cookies to enhance your experience on our website. By continuing to use our website, you are agreeing to our use of cookies. You can change your cookie settings at any time.ContinueFind out more

Each of a set of finite differences calculated by subtracting a previous value of a function f(x) from the present value when the argument x is varied by a small negative amount; frequently attributive.

Early 20th century; earliest use found in Annals of Mathematics.