真剣 [しんけん] – seriousness; a real sword
- 真 [シン・まこと] – true; reality
- 剣 [ケン・つるぎ] – sword
真剣 is a real sword. Not a toy. Not a replica. Not a wooden training tool. 真剣 is a real, live, sharp blade that can maim and kill. The notion of such a dangerous instrument gives a clear impression of something very serious.
To change this word from a noun into an adverb, simply add に: 私は日本語を真剣に勉強しました (lit – “I studied Japanese seriously” or “I made a very serious study of Japanese”). This rules works for changing many nouns into adverbs.
One of my favorite 四字熟語 is 真剣勝負 [しんけんしょうぶ] – “a fight with real swords.” It indicates that the effort being described is extremely serious, even to the point of life-and-death.