It's in the OED. I don't think it is in any American dictionary, not even the Third international. It is alleged to be the longest "general usage" (heh) word in English. It was originally concocted by a bunch of Oxford students as a sort of joke, I think in the 18th century. It means to "consider as being worthless." When I was a high school English teacher, I put it on a spelling test for extra credit. Many of the students were so taken with it that they actually learned it for the test, even some who got the simpler words wrong. Adolescent behavior is unfathomable, which is why they are so much fun to teach, especially if you have an affinity for amusing nonsense, which I do.