chaff, bran, husks of grain, (from which comes: poor, remiss)
穅 centers on the worthless outer parts of grain: chaff, bran, and husks. The extended sense of poor or remiss comes from the idea of something empty and without substance.
穅 combines 禾 (grain) with 康, which likely contributes the sound and the idea of emptiness or hollowness, reflecting the worthless husks left after threshing.
Grain 禾 beside 康 (which can suggest emptiness) gives the image of empty grain husks: chaff and bran, the worthless leftovers.
For コウ, imagine a pile of chaff so light that a single cough (コウ) sends it flying away: cough -> コウ, and the empty husks scatter.