Something a dousche bag, weak ass man says to a woman when she is shining too bright, being too happy or otherwise making him feel like the lesser he really is.
Someone telling someone else to tone it down usually has nothing to do with race, it's because the other person is full of shit.
He/she didn't like being told that he/she was full of shit even if the person that said tone it down had a point, and the one they said to tone it down to had no point, but still looked for anything he/she could to be argumentative over just to be argumentative, a stop sign being red not being good enough of an answer not to get argumentative about it without good reason, for example.