The Jargon File (version 4.4.7, 29 Dec 2003):

Mongolian Hordes technique
 n.

    [poss. from the Sixties counterculture expression Mongolian clusterfuck for
    a public orgy] Development by gang bang. Implies that large numbers of
    inexperienced programmers are being put on a job better performed by a few
    skilled ones (but see bazaar). Also called Chinese Army technique; see
    also Brooks's Law.


The Free On-line Dictionary of Computing (30 December 2018):

Mongolian Hordes technique
Chinese Army technique

    (Or "Chinese Army technique") Assigning
   a large number of inexperienced programmers to a job which
   would better performed by a few skilled ones.  The term was
   first used by Dr. Fred Brooks in his book "The Mythical
   Man-Month", Chapter 3.

   According to Dr. Brooks, he had in mind the vision of the
   Mongol Hordes sweeping across Asia and Europe when he created
   the term.

   [Jargon File]

   (1996-06-20)