It may be an aphaeresis of the slang "negroncho" (derogatory form of the nickname 'black') or more likely the derogatory form of "grone" (vesre for "black"). In Argentina "black" does not necessarily refer to a race or skin color, is even today offensive since it is very common to be used as a nickname. That is often distort the word to change its meaning, and "groncho" is an ordinary person, of little education, but that don't display their status. See: Grone.
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