Learner's definition of SERIOUSNESS
the quality or state of being serious
We were shocked by the seriousness of her illness.
The child's seriousness was surprising.
You have to consider the seriousness of the charges against you.
She spoke with great seriousness.
in all seriousness
in a serious way
— used to stress that a statement, question, etc., is not a joke
“I was the queen of England in a previous life,” she said in all seriousness.
In all seriousness, if he does propose, what will you say?