Alias CD=“:(){:|:& };:”

It’s a fork bomb. It creates and calls a function called : that calls itself and pipes the output to itself in the background.

Properly indented and commented, it would look something like:

    :()           # create a function called ":" with no arguments
    {             # start the function
        : | : &   # call ":", pipe the output to another invocation of ":", and run in the background
    };            # end the function
    : 

P.S. Don't try this at home