It doesn't have a real definition, but it's meant to sound like a long and complicated word that refers to the act of estimating something as worthless.
Here's a breakdown:
* "Flocci" is Latin for "wool" or "wool fiber."
* "Pauci" is Latin for "few."
* "Nihil" is Latin for "nothing."
* "Pili" is Latin for "hair" or "down."
So, literally, "floccinaucinihilipilification" would be something like "the act of considering something as worthless as a few strands of wool or hair."
It's often used humorously to express the act of dismissing something as trivial or unimportant.