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.