What does it mean ? Be careful to invest in the market before you can see a bottom.
I found a great definition on Investopedia:
"As the phrase suggests, buying into a market with a lot of downward momentum can be quite dangerous. If timed perfectly, a buy at the bottom of a long downtrend can be rewarding - both financially and emotionally - but the risks run extremely high. This term implies that the investment will never be a good one again. Examples of stocks that have plummeted are plentiful; a widely-held stock can drop precipitously as the equity ownership is reduced to nothing."
Source: http://www.investopedia.com/terms/f/fallingknife.asp
Photos: http://www.clker.com