In general contexts, something that ossifies becomes hardened or conventional and opposed to change. In medical contexts, something that ossifies (such as cartilage) forms or is transformed into bone.

// I asked if she'd reconsider her position, but her opinions on the matter have clearly ossified.

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"... we all probably need to be better about dealing with the gradual buildup of small annoyances in our lives before they ossify." — Paula Meja, The Atlantic, 11 Apr. 2024

The skeletons of mammals originate as soft cartilage that gradually transforms into hard bone; in humans, the process begins in the womb and continues until late adolescence. This bone-building process was given the name ossification in the late 17th century, with the verb ossify following soon after. The words come from the Latin root os, meaning "bone." (Os is also a synonym of bone in scientific contexts.) Both terms have come to refer to figurative types of hardening, such as that of the heart or mind.