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F.L.U.D.D., short for Flash Liquidizer Ultra Dousing Device, is the device used by Mario to clean up Isle Delfino in Super Mario Sunshine. It is capable of shooting water, but needs to be manually refilled. In SMG4's videos, it appears in SMG4: Stupid Mario Sunshine, which, of course, is a parody of that game. He was horrified by Mario's actions when he attempted to help others and ended up throwing Isle Delfino into chaos.
Unlike the original, Mario actually stole F.L.U.D.D from Professor E. Gadd. When he requested Mario return it, he simply squirted him away. Presumably he returned it after the events of the video.