The noun σποδος (spodos) means ash (Matthew 11:21, Luke 10:13 and Hebrews 9:13 only). In the Greek classics, this noun could describe ashes of any regular fire, or that of a votive altar or a funeral pyre. In the Greek world, ash was a sign of mourning, but it's not at all clear why that would be so — it does too in the Semitic world of the Bible, but still for unclear reasons. In Persia, ashes were used to execute people (by forcing them to breathe it in).

Our noun σποδος (spodos) could be used to describe dirt or dust on the ground. Ash was a sign of futility or nothingness, which is evidenced by the expression of figuratively reducing something to ashes, or laboring in vain. The verb σποδεω (spodeo) means to pulverize or crush. And ashes were a symbol of dryness: a drunkard might be observed to drink like ash.

Another Greek word meaning ash (not used in the New Testament) is τεφρα (tephra), which appears in the same contexts as σποδος (spodos) with no discernable difference. This second word comes from a Proto-Indo-European root "deg-", to burn. But where σποδος (spodos) comes from is a mystery. It appears to be related to words like ασβολος (asbolos), and ψολος (psolos), both meaning soot, and even σπολευς (spoleus) and σποδιτης (spodites), both descriptive of sorts of bread that were baked in hot ashes.

Here at Abarim Publications we don't know either, of course, but would like to point out that the Hebrew verb ספד (sapad) means to wail or lament. This word's root is widely attested in the Semitic language basin, and the chances are excellent that it was imported into the Greek language along with the great many other goods and services that the Phoenicians had to offer (see our article on the many Hebrew roots of Greek). Remember that the Phoenix was known to rise from its ashes (2 Corinthians 7:10).