1. one line; one stretch (e.g. of road); one strand (e.g. of hair); one beam (e.g. of light); one ray; one length (e.g. of rope)
一筋の光すら見えてこない。Not even a single ray of light is visible.
2. earnest; resolute; intent; devoted; single-minded
とにかく研究一筋ですからね、彼女は。She is single-mindedly devoted to her research, after all.
3. one bloodline; one clan
4. ordinary; common