This is the 471st most frequent Dutch word.
serieus
Serious.
Here, 'serieus' is used as an adverb to indicate the level of importance the speaker assigns to the task.
Ik neem deze taak heel serieus.
I take this task very seriously.
'Serieus' is an adjective describing the manner of joking, indicating that the jokes are not delivered in a grave or solemn tone.
Hij maakt nooit serieus grapjes.
He never jokes in a serious way.
Here, 'serieus' functions as an adjective questioning the earnestness or sincerity of the proposal.