What is "lather, rinse, repeat"

phr. (context informal often humorous English) (non-gloss definition: Indicating that an action or process is repeated.)

Lather, rinse, repeat (sometimes wash, rinse, repeat) is an idiom roughly quoting the instructions found on many brands of shampoo. It is also used as a humorous way of pointing out that such instructions if taken literally would result in an endless loop of repeating the same steps, at least until one runs out of shampoo. It is also a sarcastic metaphor for following instructions or procedures slavishly without critical thought.

It is known as the shampoo algorithm, and is a classic example of an algorithm in introductory computer science classes.

In Benjamin Cheever's novel The Plagiarist, a fictional advertising executive increases the sales of his client's shampoo by adding the word "repeat" to its instructions.