Latin phrase translation: This is most excellent work

I’d like to give a comment to a friend, in Latin, complimenting them on a recently completed project. I was thinking something like; “This is most excellent work”.

Also, is there an Latin honorific equivalent to the English “Sir” ?

optime factum [?]

ETA

Optimum opus, I think. would scan better. Hoc valde opus optimum

Do you mean “Sir” like the knightly title, or “Sir” like the honorific in “I say good day, Sir!”? I think Dominus works for the former, but seems a bit much for the latter. Maybe Magister, I think it fits better.

Wouldn’t it be the vocative domine/magistre?

Yes, if it’s being used to address the person in question, definitely. I don’t think it works like that for titles, though. So yes for my second use case, but unsure of my first.

Not so sure - the City of London motto is “Domine Dirige Nos” - does Lord in that context indicate a title of honour or simply a generic vocative? Or is it a distinction without a difference?

That’s straight vocative - In my schoolboy Latin, we always translated that as “Oh Lord” to make that clear.

But, say, “Lord Kelvin calculated …” would be Dominus Kelvin … computavit

So is what the OP wants to say.

They haven’t answered my question yet on how they want to use “Sir”

If you want to use “Sir” as an address for someone who has been knighted, then I’d say the Latin word would be eques. In ancient Rome the word originally denoted a citizen who served in the cavalry, but because of the high social standing of those soldiers, it evolved to designate a privileged class in society. So I think it’s pretty equivalent to modern-day knighthoods as it evolved from medieval chivalry. The singular word is eques, the plural equites, and in both singular and plural the nominative is identical to the vocative.

If you want to use “Sir” simply as a polite form of address for a grown-up man without indicating a knighthood, then I think dominus, vocative domine, is perfectly fine, as suggested by others.

Then again, in your Lord Kelvin example, I think it would work in Latin to address Kelvin by the actual peerage that entitled him to the address as Lord. In Kelvin’s case, he was ennobled as a baron, the Latin word for which is baro, so I think that could be used as an address. Vocative is identical to the nominative for that word.

Sorry for delay in responding. Thanks for the suggestions. I’m thinking “Sir” as the equivalent to the English “Sir Henry”, “Sir Robert”, etc. Not a peerage, just a common variety knight.

Then Dominus Henricus or Dominus Robertus would work (be sure to use the Latinized form of the name if you can, it really ups the formality - did it all the time for SCA scrolls)