This will probably sound stupid but…
What does it mean when someone replies to a thread with just nm or mm?
nm = “nevermind”
NM (Never Mind) usually means that they wrote and posted a reply, but then thought better of it (or misunderstood the question), and edited the reply to only NM.
Not sure on MM.
I thought this would be millimeters and nanometers.
As said, “nm” means “never mind.” Usually edited in when a poster thinks better of a post.
I can’t recall anybody using “mm,” but the poster Mean Mr. Mustard signs his posts “mmm.”
Otherwise “mmmmm” is something Homer Simpson says when thinking of something delicious (or sometimes just edible).
Nautical miles for me, but yes, your reaction was mine when I read the OP.
I’d guess “mm” is just fat-fingering “nm”. Easy to do on mobile devices, and spellcheck won’t save you.
Huh, I thought “nm” meant “no message”. Live and learn.
According to the Urban Dictionary, “mm” means that the poster agrees with the quote or post above. It would make more sense as “mmm”, but abbreviations!
There’s also another another definition that begins with 'mutual…" umm self pleasure. 
One reason for nm being the edit is that once posted a reply cannot be deleted and the minimum post length is two characters. I have also seen … occasionally used along with the reason Wrong thread or similar.
What does NB mean?
Poster A: [something profound]
Poster B: NB, [something equally profound]
nm
notāre (“to note”) and bene (“well”)
I’ve never seen that. More common is the useless +1.
+1
I’ve seen mm before, but it’s rare. I just assumed it meant “my mistake”.
Nanometers, 1x10 [sup]-9[/sup] meter, and millimeters, 1x10[sup]-3[/sup] meter.
In case anyone needs a clarification on this:
This board allows you to edit a post within 5 minutes, but you can’t delete a post. So if you post a reply, and then wish you hadn’t, you might delete the content and replace it with “nm”.
There was at least one place I saw where it meant that. It was somewhere that the threads displayed the subject of each individual posts before you loaded each reply, and often the subject field was long enough to post what you wanted to post. So people would do that, and courteously append “nm” to indicate that you didn’t have to load the rest of the message to see if they had another thought beyond the subject line.
I wanted to initially say it was the iMDB boards, but on retrospection I think it actually was the Hollywood Stock Exchange back in the late 90s, which is why I first thought of iMDB. They’re apparently still around although I’ve never seen anyone else ever mention them. I assumed they fell in the dot-com bubble, because I lost interest relatively quickly. I’m guessing they have a new message board system by now, but who knows?
Beginner’s Guide to Glossary of Terms Used on Straight Dope Message Boards
Linked to there is also Netlingo
The first meaning is pretty much the only one I have seen here.
Weebl and Bob too, as in “mmmmm-pie!”
I’ve edited a post down to nm when I realized it was a duplicate. I’ve also used it a few times when, after making a post, I realize that the included links are either incorrect or broken. In those cases it just seemed easier to delete, fix at leisure, and repost instead of playing beat the clock with the edit window.
I usually compose my longer posts offline as notes, then I paste them in when I’m done. So deleting and reposting is trivial.
Yes, I’ve also done it when I have regrets.