ぴかぴか

Etymology
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First attested as in the 1595 Dictionarivm Latino Lvsitanicvm, ac Iaponicvm (“Dictionary from Latin into Portuguese, and Japanese”).

Reduplication of the root of -derived verb, which root is from. See also.

As part of the reintroduction of /p/ as a phoneme, particularly among mimetic words and in some Sino-Japanese compounds, this word regained its original /p/ sound that had by then shifted to /ɸ/.

Adverb

 * 1)  glitteringly, sparklingly

Usage notes
Generally used without a particle: Sometimes encountered with the adverbial particle :

Adjective

 * 1) shiny, glossy