fossilized ない (emphatic, not negative)

Some Japanese adjectives end in ~ない without that ない being the negative marker — it's an old emphatic suffix that fossilized into the word. Examples: せわしない ('restless'), みっともない ('unsightly'), とんでもない ('outrageous'), 切ない ('painful'), 危ない ('dangerous'). Trying to negate them again produces the actual negative.