repeatedly opening and closing (one's mouth); gasping
flapping (e.g. shoe sole); gaping open and shut
heartily (eating); devouring; munching; gobbling
金魚がぱくぱく口を動かしている。
The goldfish is opening and closing its mouth
美味しそうにぱくぱく食べる。
Eating deliciously with big bites
驚いて口をぱくぱくさせた。
I gaped in surprise