Unto is an old word for 'to'. Nobody speaks this way now. You meet it in the Bible, in old poetry, in Shakespeare, in hymns, and in modern writing that wants to sound ancient or holy: 'Do unto others as you would have them do unto you.' 'And it came to pass, the Lord said unto Moses.' When it appears in modern English, it is nearly always doing one of three jobs. It is quoting or echoing scripture. It is being deliberately grand or poetic. Or it is being funny — pretending to be grand for a joke. There is also one fixed phrase that survives in normal serious writing: 'a law unto himself', meaning someone who ignores the rules everyone else follows. The Golden Rule quotation above is by far the most likely place a student will meet it, so it is worth recognising even though you will never need to write it. One practical note: unto is not always exactly 'to'. In older English it often carried a sense of 'up to' or 'as far as' — 'faithful unto death' means faithful right up to the point of dying, not faithful towards death.

（古語）向、對、至；現代已不用，多見於聖經、詩歌、莎士比亞