I've posted this question on another site and it seems to be going over well so maybe you'll enjoy it.

There are not many words in the English language, 4 letters or longer, where one letter makes up more than 50% of the word. Examples include mummy, fluff, muu-muu, puppy, and the like. The question: What is the L-O-N-G-E-S-T word you can think of that fits this bill?

Rules:
1. No proper names ("Alabama" doesn't make it)
2. No interjections ("Oooof!" and "Ahaaa!" are likewise out) and
3. Most importantly, while it is probably possible to key in a dozen or so keystrokes and have a list of applicable words spit out in five seconds, the final rule is "Grey matter only, please!"

I've got a 7-letter and a 9-letter example and someone on the other site came up with another 9-letter word (with, in this case, one letter repeated 5 times) though I'm sure there must be more.

Happy pondering!