In other words, everything I said about ba was correct. The word bă (with the “a with a hat”), pronounced something like buh or beh is different.
Bă is an interjection and roughly means “hey you”. As the commenters noted, it is identical to the word măi:
A word to address one or more males, very rarely females, the use of which denotes a conversation between two equals or that of a superior person to an inferior person; bă.
In other words, just a word to use to get the attention of a guy (or a group of guys) you are speaking to, whom you would otherwise use the tu (informal) versions of address and never the dumnevoastra (formal) forms.
A similar variant is bre, meaning the exact same thing.
While none of these words are “slang” in the purest sense, they are generally considered informal modes of speech, such as you would do with your buddies and not something you’d include in an academic paper.
AND NOW YOU KNOW!