unwahrscheinlich
unlikely
Not very likely; something whose occurrence or truth one doubts; improbable.
Es ist höchst unwahrscheinlich, dass wir im Lotto gewinnen.
It is highly unlikely that we will win the lottery.

unlikely
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unlikely
Not very likely; something whose occurrence or truth one doubts; improbable.
Es ist höchst unwahrscheinlich, dass wir im Lotto gewinnen.
It is highly unlikely that we will win the lottery.

The word is made up of 'un-' (not), 'wahr' (true), and 'scheinlich' (derived from 'scheinen'—to seem). Something that is 'unwahrscheinlich' (unlikely) therefore 'seems' 'not true' or not plausible.
Ein Sieg unserer Mannschaft scheint nach der ersten Halbzeit unwahrscheinlich.
A victory for our team seems unlikely after the first half.
The meaning depends strongly on the context. In neutral or formal sentences, it means 'unlikely'. In colloquial speech, especially before a positive adjective, it is used as an intensifier in the sense of 'unglaublich' (incredibly) or 'extrem' (extremely).
Ein Lottogewinn ist statistisch sehr unwahrscheinlich.
A lottery win is statistically very unlikely.
To express low probability, the structure 'Es ist unwahrscheinlich, dass ...' (It is unlikely that ...) is often used. The subordinate clause with 'dass' then describes the event that is unlikely.
Es ist unwahrscheinlich, dass er pünktlich kommt.
It is unlikely that he will arrive on time.
In spoken language, 'unwahrscheinlich' (incredibly) often functions as an intensifier for positive qualities. This use is informal and expresses great amazement or enthusiasm.
Ich habe gestern einen unwahrscheinlich lustigen Film gesehen.
Yesterday I saw an incredibly funny film.
probable / likely
'wahrscheinlich' and 'unwahrscheinlich' are clear opposites. 'wahrscheinlich' is used when you are more likely to expect that something will happen or is true. 'unwahrscheinlich' is used when you doubt it or are less likely to expect it. Both words fit well with guesses about the future or about a situation. What matters here is the normal meaning of 'unwahrscheinlich', not the colloquial intensifier as in 'unwahrscheinlich schön' (incredibly beautiful).

Ein Sieg ist unwahrscheinlich, weil die Mannschaft viele verletzte Spieler hat.
A victory is unlikely because the team has many injured players.
Mein Bruder kommt wahrscheinlich später, weil sein Zug Verspätung hat.
My brother will probably come later because his train is delayed.