sweeten (v.)youdaoicibaDictYouDict[sweeten 词源字典]
1550s (intransitive), from sweet (adj.) + verbal ending -en (1). Transitive sense ("become sweet") is from 1620s. The Middle English form of the verb was simply sweet, from Old English swetan. Related: Sweetened; sweetening.[sweeten etymology, sweeten origin, 英语词源]