The vocative macte is a party to a particular Latin idiom that you may encounter. Macte is the imperative of the Latin second declension adjective mactus (blessed, honored, cf. Greek μακάριος).
The idiom runs like this:
- Macte (estō) (virtūte): success attend your honor!
Now, the ‘standard’ rendering offered by A&G (^^^) is a little too translationese, in my view. Something like, ‘be blessed in honor’ would more closely attend to the syntax of each word.
Further, realize that both estō and virtūte are optional, but at least one of the two must be present (macte estō virtūte, macte estō, macte virtūte).
- With just macte! we have a different idiom all together (blessed! — something like the English ‘fantastic’ or ‘awesome’ or ‘that’s great.’)
Finally, we should echo A&G’s hesitation about the fact that the quantity of the final -e in macte is indiscernible given the extant verse poetry that contains this idiom, and therefore it might actually be mactē, an adverb. It is a matter of scholarly dispute.
The Essential AG: 340c.
Latin for Addicts 
Are there any instances where “macto” (not “macte”) is read as a frequentative form of “macio” (1st conjugation?) which means to “slice up”? Without looking it up, I think I recall a phrase from the first six books of Ab Urbe Condita (Weisenborn) that reads “macto virtute” and is translated as “kill [imperative 2nd singular] with honor.” The phrase was, I think, addressed to a patriotic patrician as he was assassinating another patrician on the grounds that the latter was buying plebian support through distributing free grain during a famine (I’m not making this up, but I haven’t looked it up, either).
I need to correct myself. I thought I had recalled the phrase as “macto virtute” from Weissenborn/Mueller’s text of Livy Ab Urbe Condita Book IV, Chapter 13 (slaying of Spurius Maelius). But the only online Latin text I can find reads, “Tum dictator, ‘macte virtute,’ inquit, ‘C. Servili, esto liberata re publica.'” I don’t have Weissenborn/Mueller’s text at hand, but I could have sworn Mueller’s text of this passage read “MACTO virtute” and Weissenborn translated it (in German) as “kill with honor,” translating both “macto” and “esto” as imperative 2nd singular. But I’ll find Weissenborn/Mueller’s text to see if it reads “macto” or “macte.”
Also, congratulations on this great blog. I wish there were something similar online for classical Greek.
Thanks! I encourage you to start that Greek blog yourself, but I’ll settle for your helpful comments on mine.