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general::
Phrase(s): stiffen something up
to make something rigid or tense. • He added a little starch to the rinse water to stiffen his collars up a bit. • The cold draft has stiffened up my neck., Phrase(s): stiffen up
to become stiff. • The bread dough stiffened up as it got cold. • My knees began to stiffen up after I sat still for an hour.
McGrawhill's American Idioms And Phrasal Verbs