Denshi Jisho — Online Japanese dictionary

ドキドキ 1:
2: (Onomatopoeic or mimetic word) thump-thump; bang-bang; pit-a-pat; pitapat; pitter-patter; (Suru verb) (Onomatopoeic or mimetic word) to beat fast (of one's heart); to throb; to pound; to palpitate
Common word, Adverb, Adverb taking the 'to' particle 

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