Here the word "heart" refers to the minds and thoughts of people. AT: "The human mind is more deceitful" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The speaker uses this rhetorical question to emphasize that no one can understand the human heart. It can be translated as a statement. AT: "no one can understand it." (See: [[rc://en/ta/man/translate/figs-rquestion]])