unüblich
unusual
Something that does not correspond to the norm, habit, or what is usual; rare or uncommon.
Es ist ziemlich unüblich, im Januar im Meer zu baden.
It is quite unusual to swim in the sea in January.

unusual
Loading your saved state.
unusual
Something that does not correspond to the norm, habit, or what is usual; rare or uncommon.
Es ist ziemlich unüblich, im Januar im Meer zu baden.
It is quite unusual to swim in the sea in January.

The word has a simple structure: the prefix 'un-' negates the adjective 'üblich' (usual, common). So if you understand 'üblich' (usual), you can easily work out the meaning of 'unüblich' (unusual).
In unserer Firma sind so kurze Kündigungsfristen völlig unüblich.
In our company, such short notice periods are completely unusual.
A very common construction with 'unüblich' (unusual) is the sentence structure with 'es' (it) and an infinitive clause: 'Es ist unüblich, etwas zu tun.' (It is unusual to do something.) You can use this pattern for many situations.
Es ist unüblich, dem Chef zu widersprechen.
It is unusual to contradict the boss.
If you want to say for whom or what something is unusual, use the preposition 'für' (for) followed by the accusative.
Ein solches Verhalten ist für einen Erwachsenen sehr unüblich.
Such behavior is very unusual for an adult.
rare / seldom
'selten' and 'unüblich' can both say that something does not occur often. The difference lies in the perspective. 'selten' emphasizes frequency: something happens only a few times or is present only in small amounts. 'unüblich' emphasizes more that something is not normal or does not fit the usual habit. Learners may confuse the words because something unusual is often also rare. But not everything that is rare is automatically unusual: a rare bird is simply not often seen, while an unusual appointment does not fit the normal routine.

Ist es nicht unüblich, einen Termin so spät am Abend zu planen?
Isn't it unusual to schedule an appointment so late in the evening?
Mein Kollege kommt selten zu spät, deshalb wundert sich heute niemand.
My colleague is rarely late, so no one is surprised today.