ない in つまらない and similar adjectives and their conjugation with すぎる

Question

The usual path for conjugation would be:

1. 高い　→　高すぎる　 (drop い, append すぎる)
   
2. ない　→　なさすぎる (drop い, append さ, append すぎる)

Is ない in つまらない treated as the negative ない which conjugates as 2.?
Or is つまらない a single piece that conjugates as 1. ?

Does つまらない conjugate to つまらなすぎる or つまらなさすぎる? Are both acceptable? If so, is there a preference for one form or the other?

Do the conclusions that apply for つまらない also apply to similar adjectives ending in ない such as:

• 危ない (ない explicit as おくりがな)
  
• 汚い (ない hidden inside the word)

Answer 1

Dictionaries say すぎる in this meaning is placed:

• after the 連用形 of a verb, like 動きすぎる
• after the stem of an i-adjective, like やさしすぎる
• and after the stem of a na-adjective, like しずかすぎる.

つまらない is an adjective so I think つまらなすぎる is natural.　

Generally, when すぎる is placed after ない:

• in the case of the adjective ない, it uses さ, like なさすぎる as you say
• in the case of the negative form of an i and na-adjective, I think it uses さ, like つまらなくなさすぎる,　しずかでなさすぎる.
• in the case of the negative form of a verb, すぎる is placed after な, like 動かなすぎる.

However, in the case where the stem of a verb is one character/mora, さ is often placed after な, like しなさすぎる,　見なさすぎる.

This さ is said to be an additional strength for the instability of one-character (single-mora) stems.

It is written here in detail: 「なすぎる」？「なさすぎる」？

In addition, ない is basically following.

・in the case of the adjective ない which means "not be" like 卵が無い.

・in the case of the adjective auxiliary ない which means "negative" and is mostly placed after adjective like 美しくない.

・in the case of a part of an adjective like あぶない.

・in the case of the verbal auxiliary ない which means "negative" like 動かない

However there are some persons who said the adjective auxiliary ない is same as the verbal auxiliary　ない because both means "negative". In detail here. http://okwave.jp/qa/q2762974.html

Answer 2

As both Yuuichi Tam and user4092 have noted in their earlier posts, the pattern for すぎる after -ない can vary. Part of this is because the -ない ending itself has two derivations.

• One is from the negative 無い. (Historically, it's more complicated, but in modern Japanese, the negative ない suffix is functionally the same as standalone 無い.) The adjective つまらない "boring" evolved from verb 詰{つ}まる, originally in reference to a story or circumstance that wouldn't つまる or finish up -- imagine someone long-winded who just won't get to the point. Adjectives like this that are based on negative 無い can often take the -さ- between the な and the すぎる.
• The other ない ending appears to derive from, or at least be cognate with, old auxiliary verb なう that expressed repetition or ongoing state, like in verbs 行{おこな}う "to carry out, to perform" or 商{あきな}う "to do business, to buy and sell". The ない in adjective 危{あぶ}ない is this other ない, not the one meaning "not". Adjectives like this generally don't take the -さ- between the な and the すぎる.

So modern つまらない can apparently form either つまらなすぎる or つまらなさすぎる, while 危ない and 汚い should only form 危なすぎる and 汚すぎる.

Note: there is some debate about this among native speakers. Textbooks make a distinction between ない on its own to describe the lack of something (like 卵{たまご}が無{な}い "there are no eggs"), which does take the -さ-, versus ない as a verbal suffix to indicate the negative, which shouldn't take the -さ-. But it seems that the people on the street might not make this same distinction, or perhaps they are making it less as time goes. Google searches do turn up examples of 危なさすぎる and 汚さすぎる, for instance, albeit with fewer hits than the expected forms without the interstitial -さ-.

See this Chiebukuro post for some discussion of the phenomenon.

Answer 3

The rule is that さ intermediates when the word stem consists of only one mora (e.g. な as in ない and よ as in よい).

The stem of つまらない is つまらな, which consists of 4 moras, therefore the orthodox one is つまらなすぎる.

That said, つまらなさすぎる is also accepted in practice. (edit: Some people probably find it wrong.) As for preference, I find both of them almost as frequent as each other.

As for 危ない and 汚い, あぶなさすぎる and きたなさすぎる are not used.