Dumping, Throwing Away
noun, verbal noun
When you throw out something and abandon it, you are essentially dumping it. 投棄 is a formal word for throwing away things, often used in the context of illegally dumping waste.
とうき
This is a jukugo word that uses the on'yomi readings of the kanji. You learned the on'yomi for 投 when you learned 投資 a long time ago.
それが不法投棄になるとは知らなかったんです。
I didn’t know it would be unlawful dumping.