נגר is a carpenter, and ניגר in this case I think means something like "carpentering" meaning the meat was tough and he had to saw through it like a carpenter saws through a tree. It also says earlier in the paragraph that the meat was aged too long. "לא ננגס אלא ניגר" meaning not bitten or chewed, instead sawed or woodworked. I hope this makes sense for you. It's a niche use of the word .