'stiffening' originates from English, specifically from the verb-forming of the adjective 'stiff' with the suffix '-en' and the gerund/nominal suffix '-ing' (i.e. 'stiff' + '-en' + '-ing').

'stiff' comes from Old English 'stīf(e)' (meaning 'rigid, hard'); the verb 'stiffen' developed by adding the verbal suffix '-en' to 'stiff', and 'stiffening' was formed later by adding '-ing' to the verb.

Initially it referred to the quality 'stiff'; over time verbal and nominal forms developed so that 'stiffen' meant 'to make or become rigid' and 'stiffening' refers to the action, process, or device that makes something more rigid — this central meaning has remained largely consistent.