it's in the episode where the eds try to make an energy drink scam and ed forgets it up. They also realize that he smells bad. So along with giving him a shower they give him a new outfit and eddy tries to train him to be cool. Edd (double d) is hyper the entire episode.
the actual quote, however, happens as Ed is standing in the middle of a typical wooden fenced alley (gg aka cartoon) and as jimmy passes by he says "Hello mister!" so Ed says "Swiggity swag, what's in the BAG?". Jimmy yells "EEEEDDDDD!" in fear and then blah whatever.
i think i may have contributed to starting the fad myself here like a month ago but now it's loving annoying.
plus it makes no sense because throughout the episode ed says various "cool" things which people should be doing instead