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

bit twiddling
 n.

    [very common]

    1. (pejorative) An exercise in tuning (see tune) in which incredible
    amounts of time and effort go to produce little noticeable improvement,
    often with the result that the code becomes incomprehensible.

    2. Aimless small modification to a program, esp. for some pointless goal.

    3. Approx. syn. for bit bashing; esp. used for the act of frobbing the
    device control register of a peripheral in an attempt to get it back to a
    known state.


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

bit twiddling

   1. (pejorative) An exercise in tuning (see tune) in which
   incredible amounts of time and effort go to produce little
   noticeable improvement, often with the result that the code
   becomes incomprehensible.

   2. Aimless small modification to a program, especially for
   some pointless goal.

   3. bit bashing, especially used for the act of frobbing the
   device control register of a peripheral in an attempt to get
   it back to a known state.

   [Jargon File]