hello, my lovelies! I found this from a song:
I'm going to guess that it's a masculine or slang form of である with the さ particle attached for emphasis (the singer is male). Just to be sure there are no other misunderstandings getting in the way though, here is my rough translation:
Telling lies is my forte
But I'm kind of bad at (telling) the truth
How my true tales are always the biggest stinking lies
So, I'm basically assuming that でさ here is just a slang way to say だ or です, but please correct me if I'm wrong.