一根筋

stubborn / inflexible / one-track minded

one / single / a (article) / as soon as / entire; whole; all; throughout / "one" radical in Chinese characters (Kangxi radical 1) / also pr. [yao1] for greater clarity when spelling out numbers digit by digit

root / basis / classifier for long slender objects, e.g. cigarettes, guitar strings / CL:條|条[tiao2] / radical (chemistry)

muscle / tendon / veins visible under the skin / sth resembling tendons or veins (e.g. fiber in a vegetable)