expression, i-adjective
Describes a person who is unshakable, confident, and not easily swayed by pressure. Often used to praise someone's mental strength or assertiveness.
彼は線の太い人だから、どんな困難にも動じない。
He's a strong-nerved person, so he won't be fazed by any difficulty.
交
A decisive attitude is required in negotiations.
The expression likely derives from the metaphor of a thick line (線) being sturdy and unbreakable, applied to a person's character. The exact origin is uncertain.