Mayst means “you may”. It is the old form used with thou, the old word for “you” when you spoke to one person. So thou mayst = you may. You will find it in the Bible, in Shakespeare, and in poems that copy that old voice: “Thou mayst as well…” meaning “you are allowed to” or “it is possible that you will”. Nobody says it in ordinary talk now. The ending -st is the old “you” ending on verbs (compare thou canst, thou hast). Though actually, students sometimes guess it means must because the letters look close — it does not. Must is a different word and a stronger meaning (“you have to”). Mayst is only the old “you may”.

（古語）thou mayst＝你可／你可以（現在一律說 you may）