"belonging to or inseparable from a disease," 1680s, from patho- "disease, suffering" + Greek gnōmōnikos "able to judge," from gnōmōn "one who knows" (see gnomon). Related: Pathognomonical (1640s); pathognomy.
"vertical shaft that tells time by the shadow it casts" (especially the triangular plate on a sundial), 1540s, from Latin gnomon, from Greek gnōmōn "indicator (of a sundial), carpenter's rule," also, in plural, "the teeth that mark the age of a horse or mule," literally "one that discerns or examines, interpreter, expert," from gignōskein "to come to know," from PIE root *gno- "to know." In geometry from 1560s, from a use in Greek. In early use in English sometimes folk-etymologized as knowman. Related: Gnomonic.
It is the hypothetical source of/evidence for its existence is provided by: Greek pathos "suffering, feeling, emotion, calamity," penthos "grief, sorrow;" Old Irish cessaim "I suffer;" Lithuanian kenčiu, kentėti "to suffer," pakanta "patience."
<a href="https://www.etymonline.com/word/pathognomonic">Etymology of pathognomonic by etymonline</a>
Harper, D. (n.d.). Etymology of pathognomonic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/pathognomonic