Source : Free On-Line Dictionary of Computing

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 mailer}s 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)