Umm, well, "n" has no meaning, unless it's used in a way that illustrates onomatopoeia (i.e. bang, crack and boom). "n" could represent grinning or grinding your teeth in that sense.
Modifying "n" with "!" :
n!
Would be a loud pronounced way of saying "n". Exclamation marks are used to "exclaim" (shout) a word or set of words.