What it means
Calling someone or something unserious means it is gloriously absurd, silly and not to be taken seriously for one second, usually said with delight at how ridiculous it is. The behaviour is a joke, in the best possible way.
Usage examples
"He showed up to the gym in a full suit, this man is unserious."
"The group chat at 2am is the most unserious place on earth."
"You wore sunglasses to a Zoom funeral rehearsal, you're so unserious."
"That little dance he did after missing the train was deeply unserious."
Where it comes from
Built from serious with the negative prefix un-, but the slang life of unserious really took off online in the 2020s, especially in Gen Z and stan-heavy spaces. It shifted from meaning simply not serious to a playful drag for people, outfits, posts, and whole situations that are ridiculously silly on purpose or by accident.
