一筋(ひとすじ)

ひとすじ
noun
one line; single-minded
descriptive
1. one line, one streak, one strand
A single thin line or strand of something, such as light, smoke, tears, or hair.
涙(なみだ)が一筋(ひとすじ)流(なが)れた。
A single tear rolled down.
A single ray of light shone into the darkness.
A single trail of smoke was rising from the chimney.
2. single-minded, devoted, wholehearted
Complete devotion to one path, pursuit, or person without wavering. Often used to describe lifelong dedication.
料理(りょうり)一筋(ひとすじ)に生(い)きてきた。
I've devoted my whole life to cooking.
He is devoted solely to judo and has no interest in other sports.
祖父(そふ)は五十年間(ごじゅうねんかん)、大工(だいく)一筋(ひとすじ)でやってきた職人(しょくにん)だ。
My grandfather is a craftsman who has been devoted to carpentry for fifty years.

一筋(ひとすじ) has two distinct uses: as a counter-like noun for thin line-shaped things, and as an expression of single-minded devotion.

Common collocations

Sense 1:

Sense 2:

Usage