What is the origin of this? What does it really mean, it doesn’t make sense to me 
You know what I mean, people say have at you before a fight. What’s that all about? I actually searched the SDMB this time, but nothing came back so, please help me with this one 
Same thing as “Put up yer dukes!”
As far as I know, which ain’t far, it first appears in the fight scene in Act V of Hamlet. Hamlet and Laertes are duelling and both get hit with a poisoned sword. Both die, Laertes “justly killed with mine own treachery” like a woodcock…
Interesting question.
The closest I could come is a speculation based on an entry in the OED (WAG Alert!).
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, . . .”
It is possible that the phrase reappeared a few times as “to have at avail,” meaning to have the advantage of, and then transferring to a call by a challenger that “I have you at avail,” leading, eventually, to a shortening in which “avail” was dropped as understood (at the time).
*Actually “Hym thought no worship to haue a knyght at suche auaille, . . .”
I’m with Bosda Di’Chi of Tricor. I would WAGuess it means ‘Have (a go) at you’.
You know, ‘to dance’, ‘get ya chum (some)’, etc.
If you feel froggy - leap.
I mostly recall seeing it in the Krazy Kay comic strips, where Ignatz Mouse would heave a brick at Krazy, yelling, “Have at you!” I think I have also seen it in 19th-century novels, usually being yelled by bar patrons at impromptu fights.
Totally uninformed etymological speculation here, but I bet it’s a corruption of some French expression for “Guard yourself, here I come.” Maybe “Sauvez vous”, or something like that.