There are two meanings:
The first is to question something unbelievable.
The second is to agree highly on something.
1. Laquita: Daquan, I'm pregnant and you ain't the papi.
Daquan: Oh 5realz?!
2. Shanaynay: Girl, Zanisha better stop talking crap or she's finna get her ass beat!
Lanishwa: Gurl 5realz tho!
2👍 1👎