early 15c., "make steadfast," from stiff (adj.) + -en (1). Intransitive sense from 1690s. Earlier verb was simply stiff "gain strength, become strong" (late 14c.). Related: Stiffened; stiffener; stiffening. Compare German steifen "to stiffen."
He stiffened when he saw his boss enter the room
stiffen the regulations
sticktoitiveness
stickum
stick-up
sticky
stiff
stiffen
stiff-necked
stiffness
stifle
stigma
stigmatic
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