To Throw Against, To Throw At
transitive verb, ichidan verb
You know that you're throwing something that is attaching to something else. How else can you say that? The best way would be to say to throw against or to throw at.
なげつける
The readings just come from two vocab words you've hopefully seen already: 投げる and 付ける. So long as you know the readings for those words, the reading for this word should be no problem for you.
人に対して小石を投げ付けるなんて、失礼だろ！
It's rude to throw pebbles at people!
父は、怒って母に風船を投げ付けた。
My father got angry and threw a balloon at my mother.
王子は、とりあえずドラキュラに玉ねぎを投げ付けてみた。
The prince tried to throw an onion at the vampire for the time being.