English▸ noun a mass of high, thick, fast-moving clouds:
there was a thin moon, a rack of cloud.
▸ verb [no object, with adverbial of direction] archaic (of a cloud) be driven before the wind:
a thin shred of cloud racking across the moon.
– ORIGIN Middle English (denoting a rush or collision): probably of Scandinavian origin; compare with Norwegian and Swedish dialect rak ‘wreckage’, from reka ‘to drive’.