thicken (v.)youdaoicibaDictYouDict[thicken 词源字典]
late 14c. (transitive), 1590s (intransitive), from thick + -en (1). Related: Thickened; thickening. An earlier verb was Middle English thick, Old English þiccian "to thicken, to crowd together."[thicken etymology, thicken origin, 英语词源]