Japanese idiom 「みずさしに」

Hello there guys, while practicing my listening comprehension doing 書き取り, I managed to find a strange idiomatic expression whose meaning is unfathomable to me.
This is an amateur'ish translation of south park to the japanese language(made by a japanese person as it seems) I found in youtube

It starts at: 1:10 and extends to 1:17 ( expression is said in 1:15, but for the sake of the context it is recommended to watch a bit longer

The speech as it seems to me

そうだったかカイル、気づくのに時間かかったぜ。カイルだけだ、おいらが嫌いだから、みずさしにアイデアぱくって

The meaning is clear and there are no major flaws to my transcription I think, however, "mizusashi ni"(If correctly written) as is implied by context, seems to be something around the lines of "being a buzzkill" or something like that, but it is just a guess.

Any help is appreciated

Back before we had a way kewl word like "buzzkill" we had to use idioms such as " throw water on xxxx", meaning to dampen enthusiasm, to put out a (figurative) fire, to rain on xxxx's parade, to p*ss on their cornflakes, etc.

Same thing.

so that is definitely a "Only Kyle would do such a thing, because he hates me, he certainly would try and rain on my parade and steal my idea" or sort of... expression

Its like the first time I'm near correct in here. Hope I can keep improving

Thank you Mike

"Rain on my parade BY stealing my idea" is more natural English for that situation, but you understand the idiomatic usage there perfectly. Good job!