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Sauce is the term 4chan users have grown accustomed to use instead of the word "source". It stems originally from a wordfilter on /b/, but gained popularity because "sauce" is more easily typed with one hand than is "source". This enables /b/tards to quickly whine for the source of an image while furiously fapping to furry porn.
Because Moot is a weeaboo the word-filter likely originated from Ghost in the Shell: Stand Alone Complex episode 9: "C: The Man Who Dwells in the Shadows of the Net – CHAT! CHAT! CHAT!" (Net no Yami ni Sumu Otoko CHAT! CHAT! CHAT! / ネットの闇に棲む男 CHAT! CHAT! CHAT!). In a virtual-reality chatroom one avatar asks for sources and another mocks them with the phrases "sarce" and "soy sauce".
"Sauce plz" is typically yelled out in ALL CAPS by leechers, usually with a pleading tone and with one or more "ZOMG"s. The expression requests anything related to the source of the previously posted pictures of bestiality lolicon, be it a torrent, a website, a Rapidshit link, or the name of the "artist"—as furry pornographers prefer to be called.
With any luck the user will fulfill the request and may even post moar pictures. At this point there is usually semen covering the pores of the right hands of the creeps who were following the thread.
Sauce whining is necessary due to the fact that 4chan addicts are completely incapable of creating original content. The pictures are seldom created by the OP, unless of course they're a grotesque mixture of hebephrenic MS Paint scrawling with Photoshop cuntpastery. The legality issues of a drawing of a furry toddler being raped by a tentacle demon are never discussed.
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