muscle, sinew, tendon, fiber, plot, plan, descent
筋 centers on the idea of a thread-like structural line: physically, it refers to muscle, sinew, or fiber; abstractly, it extends to a plot, plan, or line of descent—anything that follows a connected, logical thread.
筋 combines 竹 (bamboo) and 肋 (rib), originally depicting the fibrous, sinewy structure of bamboo and the rib-like lines of the body, together conveying the idea of muscle and sinew.
Bamboo (竹) is full of strong fibers, and ribs (肋) are surrounded by muscle and sinew. Together they make 筋: the tough, stringy stuff that holds the body together and gives structure to a story.
For キン, imagine a muscle-bound king flexing his sinewy arms: king -> キン, and his muscles are like taut fibers.
muscle; tendon; sinew
muscle
abdominal muscles; abs
spine; line of the backbone
physical strength; muscle strength
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)
lineage; stock; descent; strain; blood relationship
popular line (of products); best-selling line; hot sellers
path; route; itinerary
rebar; (iron) reinforcing bar; reinforcing steel
heart attack; myocardial infarction
reason; right
nape of the neck; back of the neck; scruff of the neck
outline; summary; gist; basic points
muscular
synopsis; outline; plot
cardiac muscle; heart muscle; myocardium
reinforced concrete; ferroconcrete
staunch; hardcore; dyed-in-the-wool
main thread (of a story)