にこにこ
にこにこ · nikoniko · JLPT N2 · adverb
smile sweetly
Sounds like nice-o nice-o: say it while you beam, the way a cat-like grin repeats
Doubled onomatopoeia: ニコ is a short, bright smile sound, and repeating it (ニコニコ) means smiling sweetly again and again. The same doubled pattern marks many Japanese expressions of steady, gentle emotion.