But sujeito also means subject does it not? A good subject to talk about for instance, makes perfect sense. Or a good subject to study, things like that. No? Even two scientists looking down at their experiment and saying "This is a good (test) subject". ...Okay I'm overdoing it. :P
The subject of a sentence or an experiment is "sujeito", that's where the colloquial form of "sujeito" came from, "the guy we are talking about", not necessarily offensive, but very impersonal therefore offensive if you are talking about someone you personally know. "Subject" translates to "assunto" when it is "the thing we are talking about".