drum, to drum, from whence comes: to rouse, to swell, to bulge
鼔 centers on a drum and the actions associated with it: drumming, rousing, swelling, or bulging like a drumhead.
鼔 is an older variant of 鼓, combining 壴 (a drum on a stand) with 攴 (a hand holding a stick), depicting the act of drumming.
The left side 壴 shows a drum on a stand, and the right side 攴 is a hand with a stick. Picture a hand striking the drum, making it swell with sound and rousing everyone.
For コ, imagine a small child drumming on a toy drum: the child says 'ko-ko-ko' as they hit it, matching the reading コ.