• 骨ほね — bone
• 骨折こっせつ — bone fracture
• 背骨せぼね — spine
• 鉄骨てっこつ — steel frame
• 骨格こっかく — skeleton, framework
• 遺骨いこつ — remains (of a body)
• 骨身ほねみ — one’s very body; flesh and bone

骨 is a compound ideogram made of 冂 (outline) + 月 (flesh/body). It represents bones inside flesh, hence the meaning “bone.”

It also extends metaphorically to mean the backbone or essence of something, as in 技のコツ (“the knack or key point of a skill”).

骨ほねを折おって家いえを建たてた。
They worked hard to build the house (lit. “broke bones”).

料理りょうりのコツを覚おぼえたいです。
I want to learn the key techniques of cooking.