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

bit bucket
 n.

    [very common]

    1. The universal data sink (originally, the mythical receptacle used to
    catch bits when they fall off the end of a register during a shift
    instruction). Discarded, lost, or destroyed data is said to have gone to
    the bit bucket. On Unix, often used for /dev/null. Sometimes amplified
    as the Great Bit Bucket in the Sky.

    2. The place where all lost mail and news messages eventually go. The
    selection is performed according to Finagle's Law; important mail is much
    more likely to end up in the bit bucket than junk mail, which has an almost
    100% probability of getting delivered. Routing to the bit bucket is
    automatically performed by mail-transfer agents, news systems, and the
    lower layers of the network.

    3. The ideal location for all unwanted mail responses: ?Flames about this
    article to the bit bucket.? Such a request is guaranteed to overflow one's
    mailbox with flames.

    4. Excuse for all mail that has not been sent. ?I mailed you those figures
    last week; they must have landed in the bit bucket.? Compare black hole.

    This term is used purely in jest. It is based on the fanciful notion that
    bits are objects that are not destroyed but only misplaced. This appears to
    have been a mutation of an earlier term ?bit box?, about which the same
    legend was current; old-time hackers also report that trainees used to be
    told that when the CPU stored bits into memory it was actually pulling them
    ?out of the bit box?. See also chad box.

    Another variant of this legend has it that, as a consequence of the ?parity
    preservation law?, the number of 1 bits that go to the bit bucket must
    equal the number of 0 bits. Any imbalance results in bits filling up the
    bit bucket. A qualified computer technician can empty a full bit bucket as
    part of scheduled maintenance.

    The source for all these meanings, is, historically, the fact that the 
    chad box on a paper-tape punch was sometimes called a bit bucket.

    [75-10-04]

    A literal bit bucket.


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

bit bucket

    1. (Or "write-only memory", "WOM") The universal
   data sink (originally, the mythical receptacle used to catch
   bits when they fall off the end of a register during a
   shift instruction).  Discarded, lost, or destroyed data is
   said to have "gone to the bit bucket".  On Unix, often used
   for /dev/null.  Sometimes amplified as "the Great Bit Bucket
   in the Sky".

   2. The place where all lost mail and news messages eventually
   go.  The selection is performed according to Finagle's Law;
   important mail is much more likely to end up in the bit bucket
   than junk mail, which has an almost 100% probability of
   getting delivered.  Routing to the bit bucket is automatically
   performed by mail-transfer agents, news systems, and the lower
   layers of the network.

   3. The ideal location for all unwanted mail responses: "Flames
   about this article to the bit bucket."  Such a request is
   guaranteed to overflow one's mailbox with flames.

   4. Excuse for all mail that has not been sent.  "I mailed you
   those figures last week; they must have landed in the bit
   bucket."  Compare black hole.

   This term is used purely in jest.  It is based on the fanciful
   notion that bits are objects that are not destroyed but only
   misplaced.  This appears to have been a mutation of an earlier
   term "bit box", about which the same legend was current;
   old-time hackers also report that trainees used to be told
   that when the CPU stored bits into memory it was actually
   pulling them "out of the bit box".

   Another variant of this legend has it that, as a consequence
   of the "parity preservation law", the number of 1 bits that go
   to the bit bucket must equal the number of 0 bits.  Any
   imbalance results in bits filling up the bit bucket.  A
   qualified computer technician can empty a full bit bucket as
   part of scheduled maintenance.

   In contrast, a "chad box" is a real container used to catch
   chad.  This may be related to the origin of the term "bit
   bucket" [Comments ?].

   (1996-11-20)