英-Don't quit your day job/heaven

1st. S1

D:Hey man, you wanna hear me sing?
A:Uh, not really.
D:Man, I've been practicing this song all week!
A:Uh, ok. I guess so.
D:tian mi mi, ni xiao de tian mi mi...What do you think?
A:Um, don't quit your day job.
D: Shut up! You're just jealous I have such a heavenly voice.(天籁之音）

Don't quit your day job is always used to diss somebody.
"Your day job" is how you make money, how you survive.

So by saying "Don't quit your day job", he's saying don't try to use singing to make money. Because if I did, try to be a singer, I would fail. In other words, he's saying I suck at singing.

If we think something is amazing, we can say "It's heavenly." We usually use this for maybe a singers voice. Oh, they sound heavenly. They sing so heavenly. Or we can use this for food, you could say "Hmm, this food tastes heavenly."