godan verb (-mu), intransitive verb
ponder deeply; brood; be lost in thought
Describes sinking into deep, often worried or serious thought, sometimes to the point of being unaware of one's surroundings. Implies a prolonged, absorbed state.
彼は難しい問題について考え込んでしまった。
He got lost in thought over the difficult problem.
Don't brood too much; let's take it easy.