expression, i-adjective
sensitive; fragile; feeble; indecisive
Describes a person who is emotionally delicate, easily hurt, or lacking in assertiveness. Often used for someone who seems physically or mentally frail, or who cannot make firm decisions.
彼は線が細いから、厳しい仕事には向いていないかもしれない。
He is sensitive, so he might not be suited for a demanding job.
People who are fragile get hurt easily over little things.
Literally 'the line is thin', an idiomatic expression comparing a person's character to a thin, easily broken line. The exact origin is uncertain, but it likely developed from the visual metaphor of a slender line lacking robustness.