In As You Like It, why does Oliver call Orlando a villain?

It is always good with such questions that ask you to
explain the meaning of a quote to look at the quote in context and see what happens
before and after it to try and see if any contextual clues will give you an idea as to
what it means. If we do this, we see that Oliver calls Orlando a "villain" in Act I
scene 1 of this play after an argument between them in which Orlando complains that
Oliver has not given him the education and upbringing that his father desired he should
have. As the argument gets gradually more and more heated, it is clear that, although
Oliver is older and has the power and money, Orlando is stronger than he is and is able
to convincingly show his superiority through fighing with him. Orlando, exasperated with
his brother and his attitude towards him, thus moves to handle him and wrestle with him.
It is at this stage that Oliver calls his brother a villain,
saying:

Wilt
thou lay hands on me,
villain?

Thus the word
indicates the distance and enmity between the brothers, introducing us with a fraternal
relationship which is in need of reconciliation, just as we will soon hear about another
fraternal relationship in need of reconciliation.