stiffen (v.)youdaoicibaDictYouDict[stiffen 词源字典]
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."[stiffen etymology, stiffen origin, 英语词源]