From the online etymology dictionary: > Master-key, one that will open ("master") a number of locks so differently constructed that the key proper to each will

From the online etymology dictionary: > Master-key, one that will open ("master") a number of locks so differently constructed that the key proper to each will open none of the others" is from 1570s https://www.etymonline.com/word/master https://www.etymonline.com/word/master