expression, i-adjective
impressive; tough; having a strong presence
Idiomatic expression describing a person or thing that has a bold, solid, or imposing quality, often used for someone with a strong character or presence. Literally 'the line is thick', suggesting robustness.
彼は線が太いから、多少の失敗ではへこたれない。
He's tough, so he won't be discouraged by a few failures.
That newcomer has a strong presence and isn't intimidated even on a big stage.
The expression likely derives from the visual metaphor of a thick line being strong and prominent, applied figuratively to a person's character or presence. The exact origin is uncertain.