
Sun dog, the phenomena of false suns which sometimes attend or dog the true when seen through a mist (parhelions). In Norfolk, a ‘sun-dog’ is a light spot near the sun, and ‘water-dogs’ are light watery clouds; ‘dog’ here is no doubt the same word as ‘dag,’ dew or mist, as “a little ‘dag’ of rain.” In Cornwall, the fragment of a rainbow formed on a rain-cloud just above the horizon is called a ‘weather-dog.’
~ Abram Smythe Palmer, Folk-etymology: A Dictionary of Verbal Corruptions or Words Perverted in Form or Meaning, 1882