"Get to work"
I think "work" in "Get to work!" is a noun, too. Consider "Get to it!"
I'm guessing that "Fanya kazi" (which, as far as I can tell would literally translate "Do work") also has the additional idiomatic meaning of "Get to work!"
I see now that "kazi" is apparently an accepted answer by itself. Could it simply be a shorter version of the expression?