means Marked by genuine weight, sincerity, or gravity — not joking, trivial, or to be taken lightly.

from From Latin 'serius,' meaning weighty or grave, which passed through Late Latin 'seriosus' and Old French 'serieux' before settling into English in the 15th century. The Latin root may be distantly related to a Proto-Indo-European notion of 'heavy' — a cousin, perhaps, of German 'schwer' (heavy, difficult) — which fits the feeling: seriousness has always carried the sense of something with mass, something you can't simply wave away.