The word 'strengthen' is a transparent English formation built upon the noun 'strength'. The core noun 'strength' itself originates from the Old English adjective 'strang' (strong), which has Proto-Germanic roots strangaz, likely connected to the idea of being tight or tense. The addition of the '-en' suffix, a productive verb-forming element in English from Old English '-nian', directly converts the state or quality of being strong into the action of making strong. Thus, the word's logic is perfectly clear: it literally means "to make or become characterized by strength."

Structure: strength (strong) + en (verb suffix) 'strength' is from Old English 'strengþu, strengþ', meaning "power, force, bodily power." The '-en' suffix is a Germanic verb-forming suffix meaning "to make or become."