The phrase "complete seriousness" means being fully focused and very serious about a particular task, without any distractions or jokes. Full definition
I explain to them — in complete seriousness, of course — that this will be the hardest test of the entire year because they have not prepared in class for the test. (educationworld.com)
Although some will want to fault him for making too little of God's justice and even wrath, still they should give patient and careful attention to his efforts to take the idea of the centrality of love with complete seriousness. (religion-online.org)
From the Nordschleife to the autobahn, an M badge on a BMW connotes one thing: complete seriousness about performance. (topspeed.com)