empty grain or rice husk, chaff
秕 centers on the worthless leftover material from grain processing: empty husks and chaff that are discarded after threshing.
秕 combines 禾 (grain) with 比, which likely contributes the sound and may suggest the idea of separating or comparing good grain from worthless husks. The exact historical development is uncertain.
The grain radical 禾 beside 比 (compare) suggests sorting grain: the good kernels are kept, and the empty husks and chaff are thrown away.
For ヒ, imagine a heap of chaff being blown away by the wind with a soft 'hee' sound: hee -> ヒ, as the worthless husks scatter.