also: せんいつ
na-adjective, noun
exclusively; with utmost care
Rare literary word meaning to do something wholeheartedly, exclusively, or with the best care. Often used in the adverbial form 専一に.
彼は研究に専一に打ち込んだ。
He devoted himself exclusively to his research.
病人の看護に専一な心がけが必要だ。
One must have a mindset of taking the best care when nursing the sick.
The word is composed of 専 (exclusively) and 一 (one), suggesting single-minded devotion. The exact historical derivation is uncertain.