三十六計

Etymology
The first mention is found in the :

The number thirty-six here is likely a figure of speech meant to denote "numerous stratagems" instead of any specific number.

On the other hand, the current  is based on a book with the same title published in 1941 that claims to be a reprint of a book discovered at a bookstall in Bin County, Shaanxi that contains the thirty-six stratagems.

Etymology
From.

Proper noun

 * 1) the