Source : Free On-Line Dictionary of Computing

dumpster diving
     
        /dump'-ster di:'-ving/ 1. The practice of sifting refuse from
        an office or technical installation to extract confidential
        data, especially security-compromising information ("dumpster"
        is an Americanism for what is elsewhere called a "skip").
        Back in AT&T's monopoly days, before paper shredders became
        common office equipment, phone phreaks (see {phreaking}) used
        to organise regular dumpster runs against phone company plants
        and offices.  Discarded and damaged copies of AT&T internal
        manuals taught them much.  The technique is still rumored to
        be a favourite of crackers operating against careless targets.
     
        2. The practice of raiding the dumpsters behind buildings
        where producers and/or consumers of high-tech equipment are
        located, with the expectation (usually justified) of finding
        discarded but still-valuable equipment to be nursed back to
        health in some hacker's den.  Experienced dumpster-divers not
        infrequently accumulate basements full of moldering (but still
        potentially useful) cruft.
     
        [{Jargon File}]