Gilbert Arenas uncorked a racial slur on Reverend Al Sharpton ... attacking him over his reaction to the police shooting of Michael Brown and the riots that have followed. Arenas posted his rant on Instagram, saying ... "the stats also show AL coon Sharpton has not helped one situation he has protested at,he actually made it worst and because of him the jury goes the other way." Gilbert appeared to be reacting to news coverage of Sharpton arriving in the St. Louis area and marching arm-in-arm with protesters. As if the racial slur wasn't enough, Arenas added, "#AL ur like a #THOT in the club,lookn for attention. What u said at trayvons rally #enoughisenough ur right were tired of u PRETENDING." Sharpton hasn't responded. Yet.