Swearing on the stew is a term that is to tell the truth, kind of like "No Cap" . This was made popular by the youtubers Miniminter, Zerkaa, Vikkstar, and Ricegum. To swear on the stew you put up right hand and make your palm face forward, then you make a fist with your left hand and say "I swear in the stew" as you move your left hand left to right.
Zerkaa: swear on the stew if your telling the truth
Miniminter:"I swear on the stew"