Guru Only Code

What is this?

(I assume it's code which relies on such obscure features of either the language or the underlying hardware that even reasonably experienced coders could not be expected to understand it simply by reading it. Thus, the code is functionally unmaintainable once the guru leaves and, again thus, it represents job security for the guru and a business risk for their employer should they, for example, drop dead one day. Quite common in the days of assembly language programming.)