鍊

Phono-semantic compound:

金 (쇠 금, “metal, iron”) — semantic, indicating relation to metalworking.

柬 (가릴 간, “select, distinguish”) — phonetic, providing the sound ryeon.

Originally referred specifically to refining or tempering metal, but the meaning broadened to include training or practicing in general.

단련 (鍛鍊) — training, discipline

수련 (修鍊) — practice, cultivation

연마·연련 (鍊磨/鍊鍊) — polishing, refining

정신 수련 (精神修鍊) — mental training, cultivation

Closely related to 練 (“to weave, to practice”), with 練 referring originally to working with thread, while 鍊 refers to working with metal. Over time, the two became interchangeable in the extended sense of “practice, training.”