"Dat ass" - is this a good thing?

Well, I'm a fitness freak you guys, and I have an Instagram where I post photos of my workout progress. Yesterday I posted a picture of my ass (not completely nude or even in undies, of course) - mostly just to show others what I've achieved. So, this guy that I really like and who I think is flirting with me commented "dat ass". Now, I have never heard anybody say that to me really, so... boys, when you say "dat ass" to somebody, what do you actually mean/what do you have in mind?
"Dat ass" - is this a good thing?
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