Going through someone else's trash looking for something valuable. Usually, this is done at corporate offices, in hope of getting confidential papers.

Back in the day, phone phreakers would go "trashing" at Bell central offices, looking for disgarded switching system manuals, technical papers, or anything else that might help them pull off the next great hack against the phone company.

More recently, Oracle employees have been accused of trashing the offices of pro-microsoft lobbying groups. Most high-tech companies now use paper shredders or a secure trash service to combat this sort of activity.