Thursday’s brutal sell-off in the stock market was one for the books.

The 4.78 percent plunge suffered by the S&P 500 index was its worst one-day drop since February 2009 and one of the worst losses (on a percentage basis) of the past 80 years.

Over the past nine days through Thursday’s close, the 500 Index has dropped 10.8 percent, making it the worst 9-day such performance since March 2009 (when, not coincidentally, the market reached a bottom).

“During the past seven days, the selling pressure on the NASDAQ has been fierce,” said Mark Arbeter, chief technical strategist at S&P.

“If we ignore the ‘flash crash’ from May 2010, the strongest down-volume versus up-volume statistics since October 2008.”

Arbeter explained that on Thursday, down-volume over up volume on the NASDAQ index was an unbelievable 85-to-1.

“This is about double the worst level of selling that we have seen in one day on the NASDAQ going back to 1978,” he said.

Similarly, for the NYSE the ratio was 90-to-1 – a figure that has only been matched or exceeded a few times going back to 1940, according to Arbeter.

“Less than 8 percent of the issues that trade on the NASDAQ rose yesterday, one of the weakest levels in the past 30 years, while less than 5 percent of issues traded rose on the NYSE, one of the worst showings in the past 40 years,” Arbeter added.