Hættu þessari vitleysu.

Hættu is the singular imperative of að hætta.

• að hætta = to stop, to quit
• hættu! = stop! / quit it! when speaking to one person

So in this sentence, hættu is a direct command to one person.

If you were speaking to more than one person, or using a formal/plural you, you would say:

• Hættið þessari vitleysu.

In Icelandic, as in English, commands usually leave out the subject.

So:

• Hættu þessari vitleysu. = Stop this nonsense.

The you is understood automatically.

You can add þú for emphasis, but it sounds more pointed:

• Þú, hættu þessari vitleysu.

That can sound annoyed, scolding, or emotionally charged.

Because að hætta often takes a dative complement/object when it means stop/quit something.

So Icelandic does not treat this exactly like a normal direct object in the accusative here. Instead, the verb governs the dative:

• hætta þessu = stop this
• hætta vitleysu would be wrong here
• hætta þessari vitleysu = stop this nonsense

This is something learners mostly have to remember as part of the verb’s usage: hætta + dative.

Because þessi changes form to match the noun it goes with in gender, number, and case.

The noun vitleysa is:

• feminine
• singular
• here in the dative

So the correct form of þessi is þessari.

A quick comparison:

• þessi vitleysa = this nonsense (nominative)
• þessa vitleysu = accusative
• þessari vitleysu = dative

Since hætta requires the dative here, you get þessari.

Vitleysa is a weak feminine noun. Those nouns often have:

• nominative singular in -a
• other singular cases in -u

So the pattern is:

• nominative: vitleysa
• accusative: vitleysu
• dative: vitleysu
• genitive: vitleysu

In this sentence, the dative is needed, so you see vitleysu.

Vitleysa means something like:

• nonsense
• foolishness
• stupidity
• ridiculous behavior

In this sentence, it is a scolding word. The speaker is saying that what the other person is doing or saying is foolish or absurd.

So the tone is stronger than something very mild like silliness, but not necessarily as extreme as the harshest insult. The exact force depends on voice and context.

It sounds fairly blunt and usually annoyed.

It is the kind of thing you might say when:

• someone is behaving foolishly
• someone keeps saying something absurd
• someone is making a situation worse

It can sound:

• firm
• scolding
• impatient

So it is perfectly natural Icelandic, but it is not especially gentle. A softer tone would usually need different wording, extra context, or a gentler voice.

Yes. Hættu is for one person.

If you are speaking to more than one person, use:

• Hættið þessari vitleysu.

So:

• hættu = singular imperative
• hættið = plural imperative

This is a very common distinction in Icelandic commands.

Yes. You can add small words to change the tone.

For example:

• Hættu nú þessari vitleysu.
  This adds nú, which often gives a sense like come on now, stop this nonsense.

• Hættu bara þessari vitleysu.
  bara can add a tone like just stop this nonsense.

But the basic structure stays the same:

• imperative verb first
• then the dative phrase

So Hættu þessari vitleysu is the clean, basic version.