Serious
noun, な adjective
The reality of the sword is that it will cut you right in half and kill you, easily. The fact that this is true is a very serious thing. Very serious.
しんけん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
売上げを損ねずに値上げする方法を真剣に考えているんです。
I’m seriously thinking about a way to raise prices without losing sales.