This is a pun on 'hardened' which in this context means having become or been made hard or harder, like what would eventually happen if you fell into wet cement. But 'hardened' can also mean experienced in a particular job or activity and therefore not easily upset by its more unpleasant aspects, like those criminals who have spent a lifetime devoted to crime
When the thief fell in the wet cement and broke both legs , he became a hardened criminal .