Let's break this up into parts. The whole clause:
means "we met/made each other's acquaintance".
Adding ばっかり to a verb in past tense means it just happened. So this means "only just met" (plus the copula だ).
まだ means "still." We've still only just met. (This is hard to translate directly into English, but it intensifies the ばっかり. まだ知り合った by itself doesn't make sense.)
Finally, the particle し just means, "and/so". I would translate the whole sentence as follows:
I mean, we've only just met, so what are you gonna do, right?
There's no negative in this sentence.