You won't fully appreciate "misunderestimate" unless you work out what it means. I daresay that I'm the first person to do so. Most people don't take the word seriously enough to make that effort. The one source I've found that attempts to define "misunderestimate" botches the task by confusing the word's meaning with the way people use it to mock President Bush. It defines "misunderestimate" as "To be excessively underestimated to the point of being ignored or ridiculed." It also, incredibly, classifies it as an adjective! I thus think I can safely discount that authority.
How to I define the word? I simply take it to combine "misunderstand" with "underestimate." One who misunderestimates his subject thus both fails to comprehend it and, consequently, to fully predict its ramifications.
As such, "misunderestimate" represents a valuable addition to the American (a label I use advisedly) language. Allow me to demonstrate how it neatly wraps two related but distinct concepts into one neat word:
Those who discount "misunderestimate" as a mere slip of the tongue misunderestimate Bush's neologism.