'Unserious' is widely used by Gen Z to describe a person, situation or take that should not be taken seriously — either because it is silly, flippant, or because it is so absurd it cannot be argued with. 'You're unserious' is often a dismissal: 'I'm not going to engage with this'. Harmless in playful contexts, dismissive in arguments. Frequent in social-media discussions of political or celebrity nonsense.