McDang has two meanings.
1st meaning of McDang: i.e. "That girl's got McDang!" Or "he gots mad McDang!" Or "that girl's runnin McDang on you son!"
2nd meaning of McDan: i.e. Hot girl walks by group of mixed people and someone shouts "McDang girl!". i.e. You find out your best friends has just screwed your girl "McDang son!!". McDang is like saying Damn! about some fine looking human, but with EXTRA emphasis. Like DAMN! x 10.