gravis

Gravis, grave, third declension, two terminations: gravis serves masculine and feminine, grave is the neuter. It is an i-stem, so the ablative singular is gravi, the genitive plural gravium and the neuter plural gravia. Comparative gravior, gravius; superlative gravissimus; adverb graviter.

Physical weight and moral seriousness sit in the same word, exactly as they do in English "grave", so for once there is no false friend to dodge. The opposite is levis, "light", in both senses too. Watch the noun family built on it: gravitas, "weight, dignity", and pondus, "weight" as a measurable quantity. English has grave, gravity, aggravate and, by a French route, grief and grieve.