the fore-arm, the upper-arm, round
厷 centers on the upper arm or forearm, with an extended sense of something round or rounded, likely from the arm's curved shape.
厷 is an ancient pictograph of an arm, with the modern form retaining a stylized arm-like shape. The exact historical development is uncertain, but it is associated with the upper arm or forearm.
The shape 厷 looks like a bent arm with a rounded elbow: the top horizontal is the shoulder, the diagonal is the upper arm, and the inner 厶 suggests the forearm curving inward. Picture a strong arm making a round, flexed shape.
For コウ, imagine a strong arm curling a heavy weight, and the effort makes you say 'ko' as you lift: ko -> コウ, and the arm's round motion matches the meaning.