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

bang path
 n.

    [now historical] An old-style UUCP electronic-mail address specifying hops
    to get from some assumed-reachable location to the addressee, so called
    because each hop is signified by a bang sign. Thus, for example, the
    path ...!bigsite!foovax!barbox!me directs people to route their mail to
    machine bigsite (presumably a well-known location accessible to everybody)
    and from there through the machine foovax to the account of user me on
    barbox.

    In the bad old days of not so long ago, before autorouting mailers and
    Internet became commonplace, people often published compound bang addresses
    using the   convention (see glob) to give paths from several big
    machines, in the hopes that one's correspondent might be able to get mail
    to one of them reliably (example: ...!seismo, ut-sally, ihnp4!rice!beta!
    gamma!me). Bang paths of 8 to 10 hops were not uncommon. Late-night
    dial-up UUCP links would cause week-long transmission times. Bang paths
    were often selected by both transmission time and reliability, as messages
    would not infrequently get lost. See the network and sitename.


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

bang path

   1.  An old-style UUCP electronic-mail
   address naming a sequence of hosts through which a message
   must pass to get from some assumed-reachable location to the
   addressee (a "source route").  So called because each hop
   is signified by a bang sign (exclamation mark).  Thus, for
   example, the path

   	...!bigsite!foovax!barbox!me

   directs people to route their mail to computer bigsite
   (presumably a well-known location accessible to everybody) and
   from there through the computer foovax to the account of user
   me on barbox.

   Before autorouting mailers became commonplace, people often
   published compound bang addresses using the   convention
   (see glob) to give paths from *several* big computers, in
   the hope that one's correspondent might be able to get mail to
   one of them reliably. e.g.

   	...!seismo, ut-sally, ihnp4!rice!beta!gamma!me

   Bang paths of 8 to 10 hops were not uncommon in 1981.
   Late-night dial-up UUCP links would cause week-long
   transmission times.  Bang paths were often selected by both
   transmission time and reliability, as messages would often get
   lost.

   2.  A shebang.

   (1998-05-06)