I think it probably derives from the production function in the "AK model" which traces back to Wicksell's 1893 "O hodnotě, kapitálu a rentě". So arguably it's a Swedish word :-) Wicksell was anti-Marx and his ideas on capital might be traced back through David Ricardo to Adam Smith's "The Wealth of Nations" i.e. English rather than German.

--------------------------------------------------
Note added at 19 hrs (2016-01-30 13:24:24 GMT)
--------------------------------------------------

Oddly enough, Wicksell did work on the Austrian School (specifically Böhm-Bawerk who was a proponent of Carl Menger's "Grundsätze der Volkswirtschaftslehre"). That school was largely non-mathematical, although simultaneous equations were at least implicit, but they tended to use "w" for Wert rather than "k" for Kapital - e.g. Menger "die Bestimmung des wirklichen Wert Maßes" (searching for Kapital in https://books.google.ie/books?id=aDTuAgAAQBAJ gives no hits but many for Werth).

--------------------------------------------------
Note added at 19 hrs (2016-01-30 13:41:07 GMT)
--------------------------------------------------

That comment isn't actually well-founded - I've totally ignored Kapital und Kapitalzins of 1884 and Positive Theorie des Kapitals of 1889.