Where did the expression "have at it" come from?

Couldn't find its etymology... anyone knows?
What does its meaning break down to?
Also, when should it be used best?

Thanks.

Solution 1:

In modern usage, to have at is to attempt, to go ahead, or to attack physically. I suspect it comes from a shortening of the phrase have a go (at), which is used in the very same situations. Have at it means try (to do) it, have at thee! announces an attack in Shakespearian English, and he had at her with a knife means he attacked her with it.

Solution 2:

Straitdope's forum suggests

The phrase "have at avail" means to have at an advantage and the earliest citation is to Malory (Le Morte D'Arthur) in the phrase* "Him thought no worship to have a knight at such avail, . . ."

Apparently "have at you" (or similar) appears in several Shakespeare plays in the sense of: let battle commence.