intransitive. To be or become hard (in various senses of the adjective).

intransitive. To become hard or harder in consistency or texture. Also with into.

intransitive. To become or remain obdurate; to harden.

intransitive. Of things: To become or grow hard.

intransitive. To become physically hard. rare.

intransitive. To become hardened or rigid. (figurative in quot.) Obsolete.