drum, beat, rouse
皷 centers on a drum and the action of beating it: a drum itself, the act of striking it, and by extension rousing or stirring up energy as if by drumming.
皷 is a variant form of 鼓, combining the drum component 壴 with 皮 (skin), likely emphasizing the drum's skin head. The exact historical development of this variant is uncertain, but it visually reinforces the drum meaning.
The left side 壴 shows a drum on a stand, and the right side 皮 means skin. Picture a drum made of stretched animal skin being beaten to rouse people.
For コ, imagine a small child (子, こ) beating a drum: 子 -> コ, and the drum's beat rouses everyone.