JSYK. Stands for “just so you’d know”. Basically, a replacement for the old familiar formal FYI. Main usage is to preface information of medium importance. It’s a recent addition. I’d say around 2006. If one can call that recent.
SMH. Stands for “shaking my head”. If you really wanna exclaim your disbelief, SMH is the way to go. I usually use it to strongly disagree. BTW, when SMH it is often thought that you are wearing a serious frown on, which essentially makes it an antonym to LOL.
IMO or IMHO, depending on how honest/humble you think you are. “in my opinion” or “in my honest opinion”. This is an oldie, been around since early 2000’s. You’d opt to use this in cases where you really think the person you’re interacting with is WRONG.
NBD. This one is “no big deal”. Which is probably why you have not seen it that often. Used to express the fact that something is not worth getting your panties in a twist. People use NP to say “no problem”, but that’s more of an agreement statement. NBD is much more dismissive. Sarcasm is always better, you know.
NSFW. This is an important one. It stands for “not safe for work”. Most times you’ll encounter this abbreviation would be just before you’ll face some obscenity, nudity, sexual content or anything else you don’t want your boss to see. Early ancestral origins is rumored to have come from Great Britain – NFBSK, which stands for “not for British school kids”.