असत्यमप्रतिष्ठं ते जगदाहुरनीश्वरम्।अपरस्परसम्भूतं किमन्यत्कामहैतुकम्॥
asatyam apratiṣṭhaṃ te jagad āhur anīśvaramaparaspara-sambhūtaṃ kim anyat kāma-haitukam
Word by word
The verse says
They say the world has no truth in it, no ground under it and nobody in charge. It came about by things bumping together. What else is there, they say, except wanting?
In plain words
This is the one verse in the chapter about a belief rather than a behaviour.
Four claims: no truth, no ground, nobody home, and all of it arrived by accident. Then the conclusion drawn from them - so, wanting. That is all there is.
Notice that Krishna does not argue with the physics. He goes straight to what the belief does to the person holding it.
Where you've felt this · in study
A boy decides at seventeen that nothing really matters. He says it as a joke at first and rather enjoys saying it. By twenty he has stopped turning up to things, because nothing is worth turning up to, and the people who kept asking have stopped asking. He has not been proved wrong about anything. He has simply arranged a life around a sentence he picked up in one afternoon.
Going deeper
A belief about the universe turning into a way of living is what is being tracked here, and the step between them happens faster than people expect.
The view described is neither unusual nor stupid. No ground, nobody in charge, things came together by chance. Plenty of thoughtful people hold a version of it.
What the Gita objects to is the sentence added at the end: so, wanting, and nothing else. That does not follow. You can think the universe is unsupervised and still think cruelty is worth avoiding, and most people who hold the first belief do exactly that.
So this is not really an argument against unbelief. It is a warning about one particular shortcut - the move from nothing is watching to therefore nothing counts. That move is not reasoning. It is permission, dressed as a conclusion, and it is usually wanted before it is argued for.