progress, busy, untiring, exert
亹 centers on sustained, diligent effort: progressing steadily, staying busy without tiring, and exerting oneself continuously.
The historical development of 亹 is uncertain. The modern form visually contains 亠 over 舋, but the original component relationships are not clearly established.
The top 亠 looks like a lid pressing down, while the bottom 舋 is a dense, busy cluster of strokes. Imagine a lid that keeps pressure on a busy workshop, forcing everyone to keep working tirelessly without pause.
For ビ, picture a tiny bee (ビ) buzzing around the busy workshop, never stopping as it exerts itself among the workers.