Neckbeards: having “facial” hair that basically only covers the neck. It’s the idea that these guys who don’t have the facial hair to grow a full beard have poor hygiene.
Incel: Involuntary Celibate, shortened to incel. Someone who can’t get laid for one reason or another. The propblem being the people who generally self-identity as incel, who are usually toxic and sexist.
These aren’t necessarily linked, but incels are usually the type to have such poor hygiene as to have a neckbeard.
One of the things I liked in the recent season of Mushoku Tensei. They’re casting fire spells in a labyrinth and they warn the MC that they’ll suffocate if they use too many.