"of or pertaining to the under world," 1882, with -ic + Latinized form of Greek khthonios "of the earth, in the earth," from khthōn "the earth, solid surface of the earth" (mostly poetic), from PIE root *dhghem- "earth."
Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).
In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit ksam- "earth" (opposed to "sky"); Greek khthōn "the earth, solid surface of the earth," khamai "on the ground;" Latin humus "earth, soil," humilis "low;" Lithuanian žemė, Old Church Slavonic zemlja "earth;" Old Irish du, genitive don "place," earlier "earth."
<a href="https://www.etymonline.com/word/chthonic">Etymology of chthonic by etymonline</a>
Harper, D. (n.d.). Etymology of chthonic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/chthonic