Nincompoopish is an adjective. Leftist/progressive arguments are nincompoopish.
Those who argue them are nincompoops. (Noun poural form)
'Face claims I'm too technical, but rules is rules.
"... to counter the leftist/progressive nincompoopishes arguments."
Should have used a comma after "leftist/progressive" to separate the 3 adjectives.
Let me correct it and tell me if it's still wrong.
First off I should not have made nincompoopish plural.
"... to counter the leftist/progressive, nincompoopish arguments."
All three adjectives, leftist/progressive and nincompoopish describe the noun arguments.
How does it look now?
"but rules is rules"
Should that be, "rules ARE rules"? :) Unless you're using "rules" as a collective noun.