"pertaining to the jaw," 1882, with -ic + Greek gnathos "jaw, cheek," properly "the lower jaw," from PIE root *genu- (2) "jawbone, chin."
Entries linking to gnathic
-ic
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.
*genu- 2
Proto-Indo-European root, probably originally "jaw, jawbone," but also forming words for "chin, cheek." It forms all or part of: chin; Compsognathus; gnathic; gnatho-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit hanuh "jaw," Avestan zanu- "chin;" Armenian cnawt "jawbone, cheek;" Lithuanian žándas "jawbone;" Greek genus "chin, lower jaw," geneion "chin;" Old Irish gin "mouth," Welsh gen "jawbone, chin;" Old English cin, "chin."