1713, "to turn into bone," a back-formation from ossification, or else modeled on French ossifier (18c.) and formed from Latin os (genitive ossis) "bone" (see osseous) + -fy. Figurative sense is from 1858. Related: Ossified; ossifying.
ossify os·si·fy (ŏs'ə-fī')v. os·si·fied, os·si·fy·ing, os·si·fies To change into bone.