halbert, arms
戟 centers on a polearm weapon: a halberd, and by extension arms or weaponry in general.
戟 combines 戈 (a halberd-like weapon) with a phonetic component on the left that likely contributed the sound. The character originally referred to a specific type of polearm.
The right side 戈 is a halberd, and the left side looks like a combination of 十 and 日, as if ten suns are mounted on a polearm. Picture a fearsome halberd with ten sun-like blades radiating power.
For ゲキ, imagine a dramatic play (劇, げき) where warriors clash with halberds: 劇 -> ゲキ, and the halberd is the star of the battle scene.