Heathcliff's origins remain a mystery. It was hinted at in the novel that he might be of "Gypsy" birth, but it's never made known. It was hinted in the 1970 film version that he was the illegitimate son of Mr. Earnshaw, but this was an invention of the films' writers, not Emily Bronte. I'm certain that if Emily ever had access to any sort of Gypsy dialect, she would not have hesitated to incorporate it into her novel. The dialect of the song fragment however, is easily ascribed to conventional, though perhaps archaic local dialect. See Boab's post for just how far-reaching (considering time and place) certain words of dialect could be. Thanks, Willa.