context:
I read this sentence in a childrens book (オー·スッパ).
Judging from the pictures, animals are eating a lemon and go crazy because of the sour taste.
The whole sentence is:
ポッカリ ういていたんです。
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Sign up to join this communityぽっかり/ポッカリ is an onomatopoeic adverb with a few different meanings.
In your context, it describes how something is floating about in water, air, etc.
(The most common usage is to describe how someone has his mouth open in a goofy-looking way.)