The word ‘serious’ has several meanings, depending on what it’s used to describe. As a result it has a bunch of opposites: frivolous, funny, non-life-threatening…

When we are grading a person’s argument or their approach to a topic for seriousness, none of those seem to capture the sort of opposite we need. And that’s where ‘unserious’ comes up. How seriously should we take this? Is it serious or unserious?

I’m not sure why there would be a relatively recent surge in its use. Etymonline cites ‘unserious’ as having 17th century origins. Ngram shows increasing usage in the modern era but starting from the 1950s onwards.

If anything it probably reflects an increased use of ‘serious’ in the context of discussing intellectual opinions, as opposed to ‘consequences’ or ‘demeanor’ or ‘injury’ – which if anything is a reflection of increased public engagement with intellectual pursuits.

To clarify, I mostly see it in political comments, as in “trump is deeply unserious as a president” (substitute trump for whoever the poster dislikes).

That’s what weirded me, without context it sounds too literary for random internet comments, but my gut feeling in a second language isn’t really worth much.