the word 'stiffen' is a transparent formation from the old english adjective stīf, which meant unbending or rigid, and the productive verb-forming suffix -en. the root stīf is of germanic origin, likely from a proto-indo-european base meaning to be rigid or compressed. the suffix -en has been a consistent feature in english for creating verbs from adjectives, such as in 'widen' or 'sharpen'. thus, the literal meaning "to make or become stiff" has remained remarkably stable from its old english components to its modern usage, which can apply both to physical rigidity and to metaphorical firmness in attitude or resolve.

Structure: stiff (rigid) + en (verb suffix) 'stiff' is from old english stīf, meaning rigid, firm. 'en' is a verb-forming suffix from old english -nian, used to create verbs meaning "to become or make something have the quality of".