キュンキュン

adverb, noun or participle which takes 'suru'

heart-wrenchingly; with a thrill in one's heart

Onomatopoeic expression for a sudden, intense feeling of emotional tightness or excitement, often romantic or cute. Used colloquially, especially in manga, anime, and social media.

• 彼かれの笑え顔がおを見みて、胸むねがキュンキュンした。

  My heart fluttered when I saw his smile.