筋
キン, すじ

muscle, sinew, tendon, fiber, plot, plan, descent

JLPT1 G6 S12 F744

Originally 竹 bamboo + a depiction of connected joints (now written 肋) → the hard portion of bamboo between the joints, used in bow-making. *Muscle*, *line*, *lineage*, and *thread/plot* (of a story or line of reasoning) derive from the distinct lines of the bamboo, with *reason*, *logic* and (information) *source* by extension. Regarding the progression from "line" to "reason/logic," compare 理.

[Full Kanji Etymology List]