From AMS Glossary
Ragged low clouds, usually stratus fractus or cumulus fractus, that occur below the main cloud base.
They are often found in the vicinity of rainshafts. Several mechanisms may explain their occurrence. They may represent rising air that has greater-than-average humidity due to the evaporation of rain or water on the ground. They may evaporate before reaching the main cloud due to mixing with the intervening dry air. Scud may also be attributed to the breakup of raindrops. The breakup fragments that are able to accumulate in regions of high humidity in rainshafts may be the source of some scud.