Not if you think in terms of Dependent Origination, in my opinion.
When a car is running and you want to switch off the engine you put a conditioned process into motion. As you turn the key you stop the sparks from igniting, which in turn stops the fuel from burning, which stops the pistons from spinning, which stops the engine. Now the car is off so it is the opposite of when the car was on, yet this was achieved through a conditioned process.
That’s what I mean by the term “opposite”.
@Bodhipaksa Anyway, one thing to consider in my opinion is the fact that the word samsara is used in the Suttas to mean the totality of transmigration for the entirety of sentient beings, it is not used individually AFAICS.
On the one hand it would not be correct to say that when one reaches Nibbana one puts and end to samsara as samsara is still there for all other beings. On the other hand it would be confusing in a teaching context to say that through reaching Nibbana one ends his/her samsara, as that would be appropriation of an impersonal process.
So, to use proper terminology for the opposite of Nibbana in an individual sense I think it’s useful to turn to Dependent Origination, where we do find Bhava which is used as opposite of Nibbana in the phrase “Nibbanam bhava nirodho”. Here it is clear that Bhava is individual as it is one of the chains of Dependent origination, while samsara isn’t.
So, in my opinion, it’s not that Nibbana and samsara are not opposites, as in the posts above the Buddha uses synonyms for these terms as opposites. But rather the two terms seem not to fit well next to each other for the reasons above.
Another thing to consider is that as you said Nibbana just means “extinguishment, quenching” and samsara means “wandering on”. The fact that we leave these terms untranslated is misleading because they would have meant very specific things in the context of the time. So saying that the quenching of a fire is the opposite of wandering in such context is just weird, and there are better ways to put it, which the Buddha used.
So the problem in my opinion is not wether the Buddha conceived of samsara and Nibbana as “opposites” or at least “different” or “contrasting”, as I think he probably did, the real question is why do we and later traditions use the terms samsara and nibbana as opposed to conditioned/unconditioned or suffering/deathless or whatever.
Personally my theory (and I think it could be interesting to investigate it) is that as tradition started to conceive of nibbana in eternalist terms (commentaries etc…), as a “realm” where arahants go when they die, then it makes sense to leave it untranslated as the name of that “realm”, and its opposite at that point becomes the name of the other “realm” that is not Nibbana, and the best name for that in the texts is samsara.
So I agree that the terms nibbana and samsara were seen differently at the time of the Buddha and therefore used in different contexts, but I don’t think that they are not “opposite” or “contrasting” or “different”, at least in a conventional sense.