Emptiness means in slang: A feeling that you are nothing or have lost your sense of self after a life-changing or traumatic event. The most common set is when a weeb finishes a lengthy anime, or a geek completes their favorite videogame. (in Slang Dictionary, added by Ricardo León)

What else does Emptiness mean?

The feeling of being empty is nothingness. It’s possible to pretend it, but really you don’t care. If you really want to be an asshole, then you are one. Although you may appear to be a good person, you really feel empty. You may feel worthless and depressed if you feel empty. (in Slang Dictionary, added by Zacarías Moya)

The feeling that there is no emotion. (in Slang Dictionary, added by London Cervantes)

This does not necessarily mean that you’re not filled with love. This means you’ve given everything. (in Slang Dictionary, added by Thomas Nicholson)