stiff•enPronunciation: (stif'un), [key]
to make stiff.
to become stiff.
to become suddenly tense, rigid, or taut, as in bracing oneself for or drawing back from shock, fear, or displeasure: He stiffened, expecting to hear the worst.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.