What is the origin of chthonian?
Chthonian ultimately derives from the Greek adjective chthónios “of the earth, the underground, the underworld.” Chthónios is a derivative of the noun chthṓn, deriving from a very, very old Proto-Indo-European word meaning “earth” and surviving in most of the “daughter” languages. The original Proto-Indo-European root was dheghm, dhghem-, dhghom-, (dh)ghm– (with various suffixes). From dheghm– Hittite derives tekan (stem tagn-) “earth,” Tocharian A (spoken in central Asia and now part of Xin Jiang) tkaṃ, Sanskrit kṣam-, and Avestan zəm-. From dh(e)ghom Greek has chthṓn, from earlier chthom (Greek also reversed the order of the consonant cluster from thch– to chth-). The suffixed form (dh)ghom-os yields Latin humus (from homos) “earth,” the adjective humilis “low to the ground” (English humble), and the noun humilitās (stem humilitāt-) “lowness of height or position, low condition (English humility). The suffixed form dhgh(e)mōn “one who is on the earth, human being” becomes hemō (stem hemōn-) in Old Latin, homō (stem homin-) in Latin. Latin also derives, somewhat obscurely, from homin– the adjective humānus “of man, human, humane, gentle” (English human and humane). (Hebrew follows a similar semantic development with ādhām “man, mankind, human being, Adam” and ădhāmāh “earth, soil, ground.”) In Germanic (dh)ghm-ōn yields guma “human being, man” in Gothic and Old English. Old English has the noun brȳdguma “young man about to be married or recently married; bridegroom, husband,” which becomes brīdgome in Middle English, and bridegroom in English. The –groom in bridegroom arose in the 16th century due to the influence of groom “boy, young man.” Chthonian entered English in the mid-19th century.