一心不乱 - JP-EN Jisho

一(いっ)心(しん)不(ふ)乱(らん)

common

adjectival nouns or quasi-adjectives (keiyodoshi)noun (common) (futsuumeishi)yojijukugo

wholeheartedly; with heart and soul; intently; with undivided attention; with intense concentration; single-mindedly