Seriousness is how grave a matter is, or how solemn a person is being. The weight of the thing.
It works on situations and on people. Of a situation it measures severity, so the seriousness of the injuries is what a doctor assesses. Of a person it describes manner, and somebody speaking with great seriousness is not joking. The phrase in all seriousness signals that a remark which might sound like a joke is not one. The adjective serious carries the same double life, meaning both grave and sincere.
1nounseriousnessThe degree to which something is grave or severe.
“Nobody grasped the seriousness of the situation.”
2nounsolemnitySolemnity of manner.
“He said it with real seriousness.”
Medieval Latin
sēriōsus
Old French
serieux
Middle English
seryows
English
serious
English, today
seriousness
From serious + -ness. serious comes from Middle English seryows, from Old French serieux, from Medieval Latin sēriōsus. From Middle English seryows, from Old French serieux, from Medieval Latin sēriōsus, an extension of Latin sērius (“grave, earnest, serious”), possibly ultimately from Proto-Indo-European swer- (“heavy”). (If this root is correct, cognate with German schwer (“heavy, difficult, severe”), Old English swǣr (“heavy, grave, grievous”). More at swear, sweer. However, see sērius for an alternative derivation of the Latin.)
Etymology from Wiktionary, CC BY-SA