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! 你戴着那种帽子出门不是开玩笑嘛!

n[U]
the quality of being serious
in all seriousness
a)spoken used to show that what you say next is not a joke, especially because it is important
= seriously
　In all seriousness, if Tom does resign, a lot of other people will start leaving too.
b) in a way that is not joking
　'Playing with Richie was the highlight of my musical career,' said Sonny in all seriousness.