← BackshareGuessultraserious/ˌʌltrəˈsɪriəs/adjectiveExtremely serious; not joking or lighthearted at all.He had an ultraserious expression when he delivered the news.The meeting was ultraserious, with no time for small talk.The ultramarathon is an ultraserious test of endurance.SynonymssolemngraveAntonymsfrivolous