'unstiffened' originates from Modern English, specifically formed from the negative prefix 'un-' + the past participle 'stiffened', where 'un-' meant 'not' and 'stiffen' meant 'to make stiff'.

'unstiffened' was created by adding the prefix 'un-' to 'stiffened' (the past participle of 'stiffen'). 'Stiffen' itself was formed from the adjective 'stiff' plus the verb-forming suffix '-en', producing 'stiffen', and then regular past formation produced 'stiffened'.

Initially, 'stiffen' and related forms referred to becoming or making rigid; the prefixed form 'unstiffened' has the transparent negative meaning 'not made stiff' or 'made less stiff', which aligns with modern usage.