The word nack abbriaviated

Ah, an intriguing query. Indeed, sir, the phrase "nack abbreviated" appears to be a playful or perhaps misheard version of "back abbreviated." If we consider this to be the case, then "nack" would be a charming little error, or perhaps a clever twist, on "back." If I may venture to suggest, the abbreviation for "back" is commonly "bk" in informal contexts. Should the visitor have intended something else entirely, I shall await further clarification with bated breath.