die Lächerlichkeit
ridiculousness
The quality of being ridiculous, absurd, or unintentionally comical and provoking mockery.
Die Lächerlichkeit seiner Ausrede war für alle offensichtlich.
The ridiculousness of his excuse was obvious to everyone.

ridiculousness / absurdity
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ridiculousness
The quality of being ridiculous, absurd, or unintentionally comical and provoking mockery.
Die Lächerlichkeit seiner Ausrede war für alle offensichtlich.
The ridiculousness of his excuse was obvious to everyone.

'Lächerlichkeit' (ridiculousness) is a noun derived from the adjective 'lächerlich' (ridiculous). The ending '-keit' is often used to form feminine nouns from adjectives that describe a quality or state.
Die Ernsthaftigkeit der Situation stand im Kontrast zur Lächerlichkeit seiner Kleidung.
The seriousness of the situation contrasted with the ridiculousness of his clothing.
The word is very often used with a genitive attribute or the preposition 'von' (of/from) to specify exactly what is ridiculous.
Die Lächerlichkeit der Situation wurde allen bewusst.
The ridiculousness of the situation became clear to everyone.
The expression 'jemanden/etwas der Lächerlichkeit preisgeben' (to expose someone/something to ridicule) is a fixed idiom. It means to deliberately expose a person or thing to mockery. The object being mocked is in the accusative.
Durch seine ungeschickte Präsentation gab er die ganze Idee der Lächerlichkeit preis.
Through his awkward presentation, he exposed the whole idea to ridicule.
joke
'der Witz' and 'die Lächerlichkeit' can both have to do with something absurd or something that cannot be taken seriously. 'der Witz' is often more concrete and colloquial: you call something a 'Witz' when it seems very bad, absurd, or ironic. 'die Lächerlichkeit' more often describes the quality of a situation or behavior: it seems ridiculous and may provoke mockery. 'Witz' more often sounds like a direct reaction in everyday life. 'Lächerlichkeit' sounds more abstract and somewhat more serious, especially when talking about the effect on others.

Mit seiner übertriebenen Ausrede machte er die Lächerlichkeit der ganzen Situation deutlich.
With his exaggerated excuse, he made the ridiculousness of the whole situation clear.
Die neue Regel im Büro ist ein Witz, weil sie niemandem hilft.
The new office rule is a joke because it doesn't help anyone.