The abbreviated [[romaji]] form of the onomatopoeia "mogu mogu," or "chew chew."

Normally written もぐもぐ or モグモグ, "mg mg" in [[romaji]] is very commonly associated with [[Ringo (touhou)|]] in [[Touhou]] 15: [[Legacy of Lunatic Kingdom]], as her official game art displays it when she is shown to be eating (which is constantly).
もぐもぐ モグモグ mgmg