unlustig
unfunny
Not funny or amusing; something that gives no reason to laugh.
Seine Witze waren so unlustig, dass niemand im Raum lachte.
His jokes were so unfunny that no one in the room laughed.

unfunny / unmotivated
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unfunny
Not funny or amusing; something that gives no reason to laugh.
Seine Witze waren so unlustig, dass niemand im Raum lachte.
His jokes were so unfunny that no one in the room laughed.

The word consists of the prefix 'un-' (not) and 'lustig' (funny/cheerful). 'Lustig' can refer to 'Witz' (joke/humor) (lustig = funny) or to 'die Lust' (desire; wanting to do something). This results in the two meanings: not funny or having no desire to do something.
Ich fand den Film überhaupt nicht lustig, sondern eher langweilig.
I didn't find the film funny at all, but rather boring.
You can recognize the meaning from the context. If 'unlustig' refers to a thing such as a joke or a film, it means 'not funny'. If it refers to a person's feeling or behavior, it means 'unmotivated'.
Seine Bemerkung sollte ein Witz sein, war aber nur unlustig.
His comment was supposed to be a joke, but it was just unfunny.
In the meaning 'not funny', 'unlustig' (unfunny) often describes things that are actually supposed to be entertaining. Typical combinations are 'ein unlustiger Witz' (an unfunny joke), 'ein unlustiger Komiker' (an unfunny comedian), or 'eine unlustige Situation' (an unfunny situation).
Er hat einen unlustigen Witz nach dem anderen erzählt.
He told one unfunny joke after another.
The meaning 'unmotivated' is common mainly in colloquial language. It is usually used predicatively to describe a person's state, e.g. 'Ich bin heute unlustig' (I feel unmotivated today).
Nach dem langen Arbeitstag war er zu unlustig, um noch ins Fitnessstudio zu gehen.
After the long workday, he was too unmotivated to go to the gym.
funny
'lustig' and 'unlustig' are opposites here. 'lustig' says that something is funny or makes people laugh. 'unlustig' says that something is not funny and is not enjoyable. 'unlustig' often sounds somewhat judgmental: you find a joke, a comment, or a situation bad or boring. Important: in other contexts, 'unlustig' can also mean 'without desire or motivation', but as the opposite of 'lustig', it is about humor.

Der Kommentar war unlustig, und niemand im Raum hat gelacht.
The comment was unfunny, and no one in the room laughed.
Der Lehrer erzählte am Ende der Stunde eine lustige Geschichte.
At the end of the lesson, the teacher told a funny story.