"Roughly" seems a good word to use.
A couple of examples from the OED:
1662 E. Stillingfleet Origines Sacræ Ded. 11 You see, Sir, to what an unexpected length my desire to vindicate [etc.].
1709 R. Steele Tatler No. 45 I heard a Voice cry, Sir, Sir!—This raised my Curiosity.
1839 C. Dickens Nicholas Nickleby viii. 69 ‘Third boy, what's a horse?’ ‘A beast, Sir,’ replied the boy.
1873 Parl. Deb. 3rd Ser. 31 July 1389/1 I rise, Sir,..to make the Indian Financial Statement.
a1930 D. H. Lawrence Phoenix II (1968) 25 Please Sir, do tortoises bite?
I'd certainly not capitalise it today, no, but I stand by my comment above that it used to be capitalised on a pretty regular basis in the olden days. I can't say I've ever seen it written in German.