1690s, "the formation of bones," from Latin ossis "of bones," genitive of os "bone" (from PIE root *ost- "bone") + -fication "a making or causing." From 1705 as "a bony formation." It is recorded earlier than ossify.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit asthi, Hittite hashtai-, Greek osteon "bone," Greek ostrakon "oyster shell," Avestan ascu- "shinbone," Latin os (genitive ossis) "bone," osseus "bony, of bone," Welsh asgwrn, Armenian oskr, Albanian asht "bone."
word-forming element meaning "a making or causing," from Latin -ficationem (nominative -ficatio), forming nouns of action from verbs in -ficare (compare -fy), combining form of facere "to make," from PIE root *dhe- "to set, put."
1713, intransitive, "to harden like bone, become bone;" 1721, intransitive, "convert to bone;" a back-formation from ossification, or else modeled on French ossifier (18c.) and formed from Latin os (genitive ossis) "bone" (from PIE root *ost- "bone") + -fy. Figurative sense "become rigid and fixed" (of thought, customs, etc.) is by 1858. Related: Ossified; ossifying.
<a href="https://www.etymonline.com/word/ossification">Etymology of ossification by etymonline</a>
Harper, D. (n.d.). Etymology of ossification. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/ossification