stumble
跌 centers on losing one's footing: stumbling or tripping. The idea is a sudden, unintended loss of balance while moving.
跌 combines 足 (foot) with 失 (lose), suggesting losing one's footing. The exact historical development is uncertain, but the component combination fits the meaning well.
The left side 足 means foot, and the right side 失 means lose. Picture a foot losing its grip on the ground, causing a stumble.
For テツ, imagine a person stumbling on a metal tetsu (鉄) plate: tetsu -> テツ, and the foot loses its footing on the hard surface.