@texczech82 the meaning of Serb is practically unknown, there are many theories both glorifying the people and degrading them. This is what wikipedia says.
Similar goes for Croats, maybe just fewer theories. wikipedia
Meaning of Pole is pretty strait forward, comes from Polans (a Slavic tribe) which comes from pole meaning field.
Czechs got their name from tribe name too. According to legends it comes from Forefather Czech, but origin is unknown.
Slovens got it same as Slovaks.
Macedonians, Montenegrins (Montenegro literally means Black Mountain), Bosniaks and Ukrainians got it after the name of the regions they inhabit.
Russians and Belarusians (Belarus means Wihte Rus) got it after old Rus, most likely of Scandinavian origin. Here is what wikipedia says about it.