adverb
eagerly; intently; single-mindedly
Describes doing something with undivided attention, without distraction or other thoughts. Somewhat literary.
彼は他念無く研究に打ち込んだ。
He devoted himself single-mindedly to his research.
子供たちは他念無く遊びに夢中になっていた。
The children were absorbed in play, without a thought for anything else.
From 他念 (other thoughts) + 無く (without). The exact historical derivation is uncertain, but the meaning is transparent from its components.