Learn Icelandic: Þetta rifrildi er bara kjánaskapur! - This argument is just foolishness!

Rifrildi (argument) is a neuter noun, so the demonstrative pronoun 'this' must take the neuter form þetta. You would use þessi for masculine and feminine nouns.

How is the word kjánaskapur built?

It combines kjáni (a fool or silly person) with the suffix -skapur. This suffix works a lot like '-ness' or '-ship' in English, turning a root word into a masculine noun that describes a state, quality, or behavior (foolishness).

Since the lesson is about possessives, how would I say 'This argument of ours'?

You would say Þetta rifrildi okkar. When you use a demonstrative like þetta (this) before a noun, the possessive usually goes after the noun (postposed). Moving the possessive in front of the demonstrative would sound very unnatural here.

Why isn't rifrildi in the definite form if we are talking about a specific argument?

In Icelandic, when you use a pointing word (demonstrative pronoun) like þetta, the noun that follows must stay in the indefinite form. It is a common mistake to try and add the definite article (like þetta rifrildið), but the demonstrative already does the job of making the noun specific!