seriousness
n [U]
1 state of being serious 严肃; 庄重; 严重; 认真
the seriousness of his expression 他的表情严肃
the seriousness of the crisis 危机的严重性.
2 (idm 习语) in all `seriousness (infml 口) very seriously; not as a joke 极严肃、 庄重、 严重或认真; 并非玩笑
You can´t in all seriousness go out in a hat like that! 你戴著那种帽子出门不是开玩笑嘛!