The term 'become hardened (to)' can be expressed in two five-letter words: 'INURE' and 'ENURE'. INURE is derived from the Latin word 'inurescere', which means to become accustomed to something, while ENURE is derived from the Old French word 'enurer', which means to become used to something. Both words refer to the process of becoming accustomed or used to something, often through repeated exposure. In this context, becoming hardened (to) refers to the process of becoming accustomed to a difficult situation or circumstance, such as a physical hardship or emotional trauma.