Early 19th century. Irregularly from Middle English sonder sunder + cloud, so called because the cirrocumulus is formed from physically separate smaller cumulus clouds.
Definition of sonder-cloud in US English:
sonder-cloud
nounˈsɒndəklaʊd
rare, historical
A cirrocumulus cloud.
Origin
Early 19th century. Irregularly from Middle English sonder sunder + cloud, so called because the cirrocumulus is formed from physically separate smaller cumulus clouds.