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.