An onomatopoeic adverb meaning 'puffed up' or a small, sudden swelling/bulging (e.g., lips, cheeks, small balloon); similar to 'pukupuku'.
虫に刺されて唇がぷくっと腫れた。
My lip puffed up after being bitten by an insect.
風船を少しだけ膨らませると、ぷくっと丸くなる。
If you inflate the balloon a little, it becomes round and puffy.
驚いて頬がぷくっと膨らんだ。
Startled, his cheeks puffed out.