Ci talk:1014647936
The ABC entry for 心根 has a note ref vkura612b6. The only reference in Wenlin's bibliography containing "kura" is Kuraishi Takeshirō. Iwanami Chūgokugo Jiten [Iwanami's Dictionary of Chinese]. Tokyo: Iwanami Shoten, 1990. And sure enough, when I checked page 612, column B of that book, there was an entry for 心根. As I suspected, however, the definition given there does not mean "secret in one's heart".
The entry reads as follows: xīngēn[r] 心根[儿] {名} 心の奥低.
The definition is written in Japanese, of course. It means "the innermost depths of one's heart". It doesn't mean "a secret" of any kind.
There are various references that say 心根 can refer to the Buddhist concept of manas (which is also translated into Chinese as 意). This is consistent with Kuraishi's definition. For example:
1) 心根 - Manas, the mind-organ, mental faculty. This term is used in both Cantonese and Mandarin/Standard written Chinese. [1]
2) 心根即为意根(manas),又称为“俱根”(ubhayendriya),它具有两种 ... (Google Books)
3) 印度古代哲学重要派别数论派的《金七十论》汉译本（真谛法师翻译）中将“manas”一词译为“心根”。如《金七十论》卷中说：“能分别为心，根说有两种者，心根有二种，分别是其体。云何如此？此心根若与知根相应，即名知根。若与作根相应，即名作根。何以故？是心根能分别知根事及分别作根事故。譬如一人，或名工巧，或为能说。心根亦如是。此心云何说为根？与十根相似。十根从转变我慢生，心根亦如是，与十根同事。”这里说的“心根”接近于佛教六根中的“意根”。 [2]
4) これからマナス（意, 心根、Manas、思考器官）、... [3]
5) "心根 Manas, or the mind-organ, one of the twenty-five tattva 諦 or postulates of a universe." A Dictionary of Chinese Buddhist Terms William Edward Soothill, Lewis Hodous [4]
Kuraishi doesn't say 心根 is a specifically Buddhist concept, though, and I've found an example (in 《二马》by 老舍) where 心根 doesn't seem to be so specialized a term as "manas":
总想忘了她，可是那里忘得下！什么事都容易摆脱，只有爱情，只有爱情是在心根上下种发芽的！ [5] This is translated by William Dolby as follows: He tried constantly to forget her, but how could he? Love's the hardest thing to get rid of, as it sprouts in the heart's deepest recesses.