I found an Old Polish word "jadro" which is "a fish net", in fact "a convexity of a fish net" (whatever this means). In other Slavic languages (Serbian and Slovenian are mentioned in my dictionary) "jadro" means (or rather meant) "a sail" and that's the original meaning of this word (it existed also in Old Church Slavonic). That's all from the most famous, though a bit outdated etymological dictionary of Polish. Maybe that's the clue? Anyway, it's more possible to be the source of the surname than "a nucleus".