That's now how it would be abbreviated; it would be m-N.
I had meant to respond to this before, but then forgot. It's almost not worth pointing this out since it's obvious to all of us from everyday experience, but since we are in a student's thread, it doesn't hurt to point out something here that might stick in PG's mind in years to come. The usual convention I've seen conforms to the reference below. Typically in papers, or when writing by hand, it's good practice to use a raised dot, and not a dash. I agree, if one does this diligently, then there is no problem. Personally, I always use dots in anything I do by computer. This applies to variables in equation and for units too. I try to do this in hand-written derivations also, but it's very hard to be diligent and consistent when working fast, so I often don't do it and just try to be careful. Ideally, one should always use raised dots to avoid ambiguity. For example, how do you know f(t-τ) is not f times t-τ, instead of the function f(t-τ), unless one uses dots. Often, people use spaces, so f(t-τ) and f (t-τ) are different, and mN and m N are different.
http://www.poynton.com/notes/units/
The problem is that (especially in hand written derivations) people either get lazy or get absent-minded, and forget to put the dot or don't leave enough space. Then, the author or another reader might misinterpret. Often context clarifies any ambiguity, but it's good to have a system that minimized possible ambiguities. For this reason, I would prefer N m over m N (or even m-N) for torque units.