Other forms: stiffened, stiffening, stiffens
To stiffen is to go rigid, usually from tension. He stiffened, stiffen at the news, stiffened with fear.
In a person it is involuntary and readable — stiffening gives away alarm or offence before anything is said. Of materials it means becoming less flexible, and of a mixture it means thickening. To stiffen resolve is to strengthen it. Note the -en ending, which makes verbs of adjectives as in harden.
1verbstiffenTo go rigid, usually from tension.
“She stiffened when she heard footsteps behind her in the dark street.”
stiffen is a going rigid.
Middle English stifnen is stiff plus -en, an ending making verbs of adjectives. Harden and soften are formed the same way.
Middle English
stifnen
English, today
stiffen
From Middle English stifnen, equivalent to stiff + -en.
Etymology from Wiktionary, CC BY-SA