There wasn't anything that would qualify as NSFW

I've seen memes that aren't marked that are obvious nsfw

Don't want to sound like a newb, but what is NSFW?

Stands for "not safe for work"
It's used to mark inappropriate content

I don't want to feel some stranger in Imgflip but how to mark a meme NSFW?

I don't believe you can mark your own creations. When you're on a meme page there is a flag near where it shows the creators name. You would click that and choose the reason for flagging it, which would be NSFW.