mortmain (n.)youdaoicibaDictYouDict[mortmain 词源字典]
"inalienable ownership," mid-15c., from Anglo-French morte mayn, Old French mortemain, literally "dead hand," from Medieval Latin mortua manus; see mortal (adj.) + manual (adj.). Probably a metaphorical expression.[mortmain etymology, mortmain origin, 英语词源]