an extensive group of cirrocumulus or altocumulus clouds, especially when well-marked in their arrangement: so called because of a resemblance to the scales on a mackerel.
Origin
1660-70
British Dictionary definitions for mackerel sky
mackerel sky
noun
1.
a sky patterned with cirrocumulus or small altocumulus clouds
Word Origin
from the similarity to the pattern on a mackerel's back