1.
one line, one streak, one strand
A single thin line or strand of something, such as light, smoke, tears, or hair.
涙が一筋流れた。
A single tear rolled down.
暗闇の中に光が一筋差し込んだ。
A single ray of light shone into the darkness.
煙突から煙が一筋立ち上っていた。
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.
料理一筋に生きてきた。
I've devoted my whole life to cooking.
彼は柔道一筋で、他のスポーツには興味がない。
He is devoted solely to judo and has no interest in other sports.
祖父は五十年間、大工一筋でやってきた職人だ。
My grandfather is a craftsman who has been devoted to carpentry for fifty years.
一筋 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:
- 涙が一筋 (a single tear)
- 光が一筋 (a ray of light)
- 煙が一筋 (a trail of smoke)
- 希望の光が一筋 (a glimmer of hope)
Sense 2:
- 〜一筋に生きる (live devoted to ~)
- 〜一筋でやってきた (have been devoted to ~ all along)
- 一筋の道 (a single path, unswerving dedication)
USAGE NOTES:
- In sense 2, 一筋 often follows the noun it modifies directly: 野球一筋 (devoted to baseball)
- 一筋縄ではいかない is a set expression meaning "not straightforward" or "not easy to deal with"