Beefiest tuna means that a girl has a fat pussy. Meaning she is a notorious slut, and her pussy is big. The pussy is extremly fat. If a girl has a beefy tuna, the pussy is fistable
Guy 1: Hey Guy 2 did you know that the girl has beefy tuna!
Guy 2: Yea dude i heard she had the beefiest tuna you could get.