Beauteous means beautiful. That is the whole meaning — but not the whole story of how it feels. It is an old, poetic adjective. You will find it in Shakespeare and in high, old-fashioned verse; you will almost never hear it in a shop, a text message, or a normal conversation. If you call a person beauteous today, you sound as if you have stepped out of a play. Beautiful is the living word; beauteous is the same idea in a costume. Teachers may still like it when you meet it in a poem, because it keeps the rhythm or the old voice. Outside literature, leave it on the page.