dust
霢 centers on fine, mist-like dust or drizzle. The given meaning 'dust' likely refers to airborne particles, possibly linked to light rain or mist.
霢 combines 雨 (rain) with 脈 (vein/pulse), likely as a phono-semantic compound where 脈 hints at the sound and the idea of fine, thread-like particles. The exact historical development is uncertain.
Rain 雨 falls in fine, vein-like 脈 streams, like dust motes or misty drizzle. Picture a dusty haze suspended in the air after a light rain.
For バク, imagine a dusty back road after a drizzle: back -> バク, and the fine dust clings to everything.