Why is こころ + みる the word used to say "to try/attempt"? What is it supposed to be saying as an idiom?
When I did a sentence search on jisho.org for こころみる, there were several sentences for the 試みる version and none for the 心見る version, suggesting that it is likely antiquated to the point of being irrelevant in modern Japanese. In other words, if it's not a usage that you're likely to see or to use, does it matter where it came from? But I guess if you're just wondering for the sake of curiosity, perhaps a native speaker with a deep knowledge of outdated kanji could explain.