I'm looking for the longest English word that has no variants, where a variant might be
- A singular or plural form
- A conjugated form
- A form in another part of speech
mouse would fail by all those rules: it has a plural ("mice"); as a verb, it has conjugated forms ("mousing", "moused"); it has an adverbial form ("mousy") and other noun forms ("mouser").
Obviously, pretty much any verb would be out, as would any countable noun that has a plural. I'm also excluding words that are just conglomerations of other words like "whatsoever".
Right now, I've got some five-letter word -- "moose", "there" -- and "through", which has seven but I'm thinking should be excluded because of "throughway" and "throughput".