the word 'toughen' is a transparent english formation, combining the native adjective 'tough' with the productive germanic suffix '-en'. the adjective 'tough' itself has a long lineage, deriving from old english 'tōh', which meant firm, tenacious, or difficult to break, a sense it has retained. the suffix '-en' is a classic english verb creator, attached to adjectives to indicate the process of acquiring that quality. therefore, the literal construction "to make/become tough" maps directly and logically onto its modern meanings, which involve the process of physically hardening something or metaphorically strengthening character or resilience.

Structure: tough (strong, durable) + en (verb-forming suffix) tough: from old english 'tōh' meaning strong, tenacious; of germanic origin. functions as the adjective root conveying the core meaning of strength and resilience. en: a verb-forming suffix of germanic origin, used to create verbs from adjectives or nouns, meaning "to become" or "to make".