To "juice up" something means "to add more power, energy, or speed to; accelerate". The name "Juiced.GS" is derived from one of founding editor Max Jones' friends, who, upon seeing Max and knowing of his decked-out Apple IIGS system, loaded with peripherals, exclaimed "There's the guy with the juiced GS!"
You can read more of our history on Dr. Steven Weyhrich's Apple II History website.