What Does It Mean?

You are correct!

From our dictionary:
os•si•fy

(os'u-fī"),
v., -fied, -fy•ing

v.t.
to convert into or cause to harden like bone.

v.i.
1. to become bone or harden like bone.
2. to become rigid or inflexible in habits, attitudes, opinions, etc.: a young man who began to ossify right after college.