Have at thee

Have at thee.
What is this phrase actually saying, and what are its origins?

THE HISTORY OF KING HENRY THE SIXTH, THIRD PART
by William Shakespeare

CLIFFORD.
Now, Richard, I am with thee here alone.
This is the hand that stabbed thy father York,
And this the hand that slew thy brother Rutland;
And here's the heart that triumphs in their death,
And cheers these hands that slew thy sire and brother
To execute the like upon thyself;
And so have at thee!

[They fight. Warwick enters; Clifford flies.]

I assume it means the same as "I'll have you!" meaning "I'll fight you!"

Pamela

The expression is old enough to appear in Middle English. Here's what the OED has to say, along with a few examples from before Shakespeare's time.

"20. . . . have at: To go at or get at, esp. in a hostile way; to have a stroke at, make an attempt at. . . .
{Examples:] 13[00s] Gaw. & Gr. Knt. 2288 'Haf at þe þenne, quod þat oþer. c1385 CHAUCER L.G.W. 1383 Hipsiphile, Haue at the Iason now thyn horn is blowe. a1529 SKELTON Bowge of Courte 391 Have at all that lyeth vpon the burde! . . ."

Why "have" instead of "go"? The OED, as usual, does not try to explain.
SS