Literally, ponele could mean put to it, put to him or put to her. Yeah, it makes no sense. In Spanish doesn’t either.
[For grammar geeks only: poner is in Spanish, such as in English (to put), a transitive verb. That means it needs an accusative particle or direct object: when you put, you need to put SOMETHING, right? Well, that’s the object we’re talking about. However, this little word, ponele, has 2 parts: the verb poner in its imperative form (poné) and a particle at the end, which is a dative particle or indirect object, not an accusative one. So, grammatically speaking, this expression is incorrect. When the speaker says ponele, he or she is literally ordering you to put nothing to something or someone. Ridiculous, right?]
But then, what does ponele REALLY mean in Argentinean slang?
In very general terms, we could say it’s a semi-ironic way of answering to a yes/no question: when you say it, you’re kind of granting a yes, but there’s some irony in it. However, it’s usually not completely ironic. When you say ponele, you’re generally doubting about what your interlocutor is saying, but not disapproving it completely.
To keep something of its literal translation, we could say ponele means something like yeah, you could put it in those terms. But, actually, it’d be more something like yeeeeaaaah, you could put it in those terms… maybe.
Let’s see a few examples:
A – El novio de Juana es simpático, ¿no?
[Juana’s boyfriend’s cool, isn’t he?]
B – Eeh… ponele.
In this case, B might not have liked Juana’s boyfriend that much…
A- Podríamos ir a comer afuera hoy, ¿no?
[We could go out to eat today, couldn’t we?]
This case is different: here, B actually thinks having dinner in a restaurant might be a good plan, but he’s not 100% sure about it… or maybe he’s just not paying much attention to A because she/he’s doing something else or because they’re still several hours away from dinner.
Ponele, second level
Another description of what a speaker could mean when he or she uses this expression could be: Let’s pretend what you say it’s truth, I concede you that, but let’s also remember I’m not that sure about it. In this last description, we can highlight there’s an invitation to the other person to keep talking, but there’s also a warning: we’ll go back to that later / I’m granting you that now, but wait for my answer / We can pretend I agree with that but I want to check where you’re going before I do admit I agree with you. For example:
A- A vos te encanta la comida vegetariana, ¿no?
[You love vegetarian food, right?]
B- Ponele. ¿Por qué?
B does like veggie food. Maybe he/she actually loves it. However, he/she’s on the defensive. He or she might be afraid A want him/her to become a vegetarian, so he/she wants to know why A is asking before fully conceding a “yes”.