Dumpster diving is the practice of sifting through commercial or residential trash

Waste

Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...

 to find items that have been discarded by their owners, but that may be useful to the dumpster

Dumpster

A dumpster is a large steel waste receptacle designed to be emptied into garbage trucks. The word is a genericized trademark of Dumpster, a American brand name for a type of mobile garbage bin...

 diver.
The dumpster diving term originates from the best-known manufacturer of commercial trash bins, Dempster, who use the trade name "Dumpster

Dumpster

A dumpster is a large steel waste receptacle designed to be emptied into garbage trucks. The word is a genericized trademark of Dumpster, a American brand name for a type of mobile garbage bin...

" for their bins, and the fanciful image of someone leaping head first into a dumpster as if it were a swimming pool.