A rather rude retort; the insinuation being that theintended audience of this request shove an object of questionable safety & hygiene sideways up their bum.
Teacher to pupil: "Chloe, i hear you flapping your busy little gums back there. Since you're currently receiving a "D" in this class, i'd think you - OF ALL PEOPLE!- would pay attention".
Chloe to teacher: "Shove it sideways, ya old bag!"
Teacher: "Whaaaaa!!?!"
Basically a quick way of suddenly interrupting someone and telling them to shut the fuck up--because you don't want to hear whatever it is they are lying about, bragging about, or complaining about. Instantly makes them realize you know they are full of shit and they stop.