trample, tread, kick
踹 centers on forceful contact with the foot: trampling, treading heavily, or kicking. The meanings share the idea of pressing or striking downward or outward with the sole.
踹 combines 足 (foot) with 耑, which likely contributes the sound and may relate to an older meaning of stepping or pressing. The exact historical development is uncertain.
The left side 足 means foot. Picture a foot stomping down on the right side 耑 like a heavy boot trampling and kicking.
For セン, imagine a centipede being trampled underfoot: cent -> セン, and the foot kicks it away.