jellyfish
蜇 centers on a jellyfish, a stinging sea creature. The meaning is specific to this animal, with no broader conceptual cluster.
蜇 combines 虫 (insect/creature) with 折, which may contribute a phonetic element. The exact historical development is uncertain, but the character likely arose to name the jellyfish.
The bottom 虫 suggests a sea creature, and the top 折 looks like something folded or bent. Picture a jellyfish with folded, trailing tentacles: 蜇 is that jellyfish.
For テツ, imagine a jellyfish made of iron (鉄, てつ) drifting in the sea: iron -> テツ, and its metallic sting is unforgettable.