conjugations - Why is it なさそう and not なそう

"It seems there is none" is なさそう, which escapes the usual rule for 形容詞 (イ-adjectives), which says "drop the い and add そう".

Is there a historical explanation for this exception? And does it have anything to do with なさいません?

Answer

Your "usual rule" is incomplete. It should be:

1. drop -i


2. if resulting is a single mora in length, add -sa


3. add -sou.

Hence, nai:

1. na


2. na + sa


3. na + sa + sou --> nasasou.

atui:

1. atu



2. (not applicable)


3. atu + sou --> atusou.