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