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

DEADBEEF
 /ded?beef/, n.

    The hexadecimal word-fill pattern for freshly allocated memory under a
    number of IBM environments, including the RS/6000. Some modern debugging
    tools deliberately fill freed memory with this value as a way of converting
    heisenbugs into Bohr bugs. As in ?Your program is DEADBEEF? (meaning
    gone, aborted, flushed from memory); if you start from an odd half-word
    boundary, of course, you have BEEFDEAD. See also the anecdote under fool
    and dead beef attack.


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

DEADBEEF

    /ded-beef/ The hexadecimal pattern
   used to fill words of freshly allocated memory under a number
   of IBM environments including the RS/6000; equal to
   decimal 3,735,928,559 (unsigned) or -559,038,737 (32-bit
   signed).  As in "Your program is DEADBEEF" (meaning gone,
   aborted, flushed from memory).

   (1998-06-29)