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

bit bang
 n.

    Transmission of data on a serial line, when accomplished by rapidly
    tweaking a single output bit, in software, at the appropriate times. The
    technique is a simple loop with eight OUT and SHIFT instruction pairs for
    each byte. Input is more interesting. And full duplex (doing input and
    output at the same time) is one way to separate the real hackers from the 
    wannabees.

    Bit bang was used on certain early models of Prime computers, presumably
    when UARTs were too expensive, and on archaic Z80 micros with a Zilog PIO
    but no SIO. In an interesting instance of the cycle of reincarnation,
    this technique returned to use in the early 1990s on some RISC
    architectures because it consumes such an infinitesimal part of the
    processor that it actually makes sense not to have a UART. Compare cycle
    of reincarnation. Nowadays it's used to describe I2C, a serial protocol
    for monitoring motherboard hardware.


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

bit bang

   Transmission of data on a serial line accomplished by
   rapidly changing a single output bit, in software, at the
   appropriate times.  The technique is a simple loop with eight
   OUT and SHIFT instruction pairs for each byte.  Input is more
   interesting.  And full-duplex (doing input and output at the
   same time) is one way to separate the real hackers from the
   wannabees.

   Bit bang was used on certain early models of Prime
   computers, presumably when UARTs were too expensive, and on
   archaic Zilog Z80 micros with a Zilog PIO but no SIO.  In
   an interesting instance of the cycle of reincarnation, this
   technique is now (1991) coming back into use on some RISC
   architectures because it consumes such an infinitesimal part
   of the processor that it actually makes sense not to have a
   UART.

   [Jargon File]