The adjective form is nonsensical, itself a slightly nonsensical word.
The word nonsensical *is* an adjective. It is the adjective form of the noun nonsense.
The noun form for the adjective authentic is authenticity.
Nonsense is a mass noun. It has no plural form.
The noun form of the adjective 'dangerous' is dangerousness.The word 'dangerous' is the adjective form of the noun danger.The noun form of the adjective 'childish' is childishness.The word 'childish' is the adjective form of the noun child.The noun form of the adjective 'beautiful' is beautifulness.The word 'beautiful' is the adjective form of the noun beauty.
The noun form of the adjective 'wide' is wideness.A related noun form is width.
The noun form of the adjective 'comfortable' is comfortableness.The word 'comfortable' is the adjective form of the noun comfort.
The abstract noun form of the adjective unique is uniqueness.
The abstract noun form for the adjective convenient is convenience.
The noun form for the adjective hilarious is hilariousness. A related noun form is hilarity.
The noun form of the adjective 'modest' is modestness.A related noun form is modesty.
The noun form of the adjective close is closeness.
The adjective form for the noun nation is national.