When someone or something is so extremely aggravating that it's just not enough to call them an annoyance or a nuisance, so you use a portmanteau of both.
Jeff: My little brother is getting on my damn nerves, he's so annoying.
Mark: My sister is even worse, she's the literal definition of annuisance.
“Unnus Annus” meaning “One year” in Latin. This is an inside joke made by the Markiplier and Crankgameplays, the men creating a seemingly cult like base, again.
“Unnus Annus” chanted Ethan, staring listlessly into the audience. “One year” added Mark, expressionism blank.