Mind monkey or monkey mind, from Chinese xinyuan and Sino-Japanese shin’en 心猿 [lit. “heart-/mind-monkey”], is a Buddhist term meaning “unsettled; restless; capricious; whimsical; fanciful; inconstant; confused; indecisive; uncontrollable”
In addition to Buddhist writings, including Chan or Zen, Consciousness-only, Pure Land, and Shingon, this “mind-monkey” psychological metaphor was adopted in Taoism, Neo-Confucianism, poetry, drama, and literature
“Mind-monkey” occurs in two reversible four-character idioms with yima or iba 意馬 [lit. “thought-/will-horse”], most frequently used in Chinese xinyuanyima 心猿意馬 and Japanese ibashin’en 意馬心猿. The “Monkey King” Sun Wukong in the Journey to the West personifies the mind-monkey. Note that much of the following summarizes Carr (1993).