It's funny to me because I've never heard anyone refer to Nintendo as "Big N" there's a handful of other things referred to as Big N, Nintendo is not one of them.
I replied in another thread [1] but this was the earliest thing I could find in online resources, but maybe your recollection would lead to something earlier:
"Nickname for Nintendo"
I remember it being frequently used as far back as the 80s by Electronic Gaming Monthly