seriousness
n [U]
1 state of being serious 嚴肅; 莊重; 嚴重; 認真
the seriousness of his expression 他的表情嚴肅
the seriousness of the crisis 危機的嚴重性.
2 (idm 習語) in all `seriousness (infml 口) very seriously; not as a joke 極嚴肅、 莊重、 嚴重或認真; 并非玩笑
You can't in all seriousness go out in a hat like that! 你戴著那種帽子出門不是開玩笑嘛!