A spiffy-drop is when a stock drops more dollars per share in a single day than you paid in your own cost per share. It is the opposite of a spiffy-pop.
So for example, if originally you paid $10.15 a share for your shares of Yahoo, and then on some horrible future day Yahoo stock drops $10.16, you have just suffered an ignoble spiffy-drop.
It's probably worth pointing out that a spiffy-drop usually only happens to an investment that has done well. The ancient Norse saying that "it takes a spiffy-pop to eventually make a spiffy-drop" is more often true than not.
This term was coined in the Motley Fool Rule Breakers community, as an elegant mot juste complementary to the Rule Breaker invention of spiffy-pop.