"powdery remains of fire," Old English æsce "ash," from Proto-Germanic *askon (source also of Old Norse and Swedish aska, Old High German asca, German asche, Middle Dutch asche, Gothic azgo "ashes"), from PIE root *as- "to burn, glow." Spanish and Portuguese ascua "red-hot coal" are Germanic loan-words.
An ancient symbol of grief or repentance; hence Ash Wednesday (c. 1300), from custom introduced by Pope Gregory the Great of sprinkling ashes on the heads of penitents on the first day of Lent. Ashes meaning "mortal remains of a person" is late 13c., in reference to the ancient custom of cremation. Meaning "Finely pulverized lava thrown from a volcano" is from 1660s.