se ‧ ri ‧ ous ‧ ness /ˈsɪəriəsnəs, ˈsɪəriəsnɪs $ ˈsɪr-/ BrE AmE noun [uncountable]
[ Word Family: noun : ↑ seriousness ; adverb : ↑ seriously ; adjective : ↑ serious ]
1 . the quality of being serious
2 . in all seriousness
a) spoken used to show that what you say next is not a joke, especially because it is important SYN 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.