Means hello. Comes from the word Tag in German, also meaning hello. Nyaig is said in a devilish, impish manner and is usually said with a scrunched up face which supposidly replecates a devils face.
2 guys meet in a bar, 1 of the guys is stood at the bar with a pint for himself and a pint for his mate. The guy who walks up to the bar 2nd, taps his mates shoulder and says, in impish manner....nyaig!!!!!!