iron, strong, solid, firm
銕 centers on the qualities of iron: something physically strong, solid, and firm. The meanings extend naturally from the metal itself to its characteristic properties.
銕 is a variant form of 鉄 (iron). It combines 金 (metal) with 夷, which likely contributes a phonetic element. The exact historical development of this variant is uncertain.
The left side is metal 金, and the right side 夷 looks like a sturdy, solid structure. Picture a block of iron so strong and firm that nothing can break it.
For テツ, imagine testing the iron's strength with a heavy metal test: test -> テツ, and the iron proves solid and firm.