Indeed. Then they were called Bezjaki or just Provincial Croatians I believe.
In my area we use "krumpľe"
I believe it's common for Southern Slavs to call Germans "Švabi". At least we do. And I also think one of the Serbian names for cockroaches is Bubašvaba (I always imagine a "German bug" under this name ).
Anyhow, we still didn't get to the bottom of our primar question. Why kaj? What's it's origin? Anyhow, kaj wouldn't be the first funny inovation of Slovene and Kajkavian.