Literally: mind – horse – heart – monkey
Alternately: This Buddhist phrase expresses how it is difficult to control oneself and detach from the illusory material world because worldly desires, false beliefs, and mental pollution such as hatred all get in the way. The mind is like a runaway horse; the heart is like a wild monkey.
Notes: This is a compound of compounds, and while the version above is more common, the halves can be switched to give 心猿意馬.
My sources give a number of, um, sources for this particular compound. The most frequent is as an annotation to a text known as 『参同契』 (Sandoukei or Sandoukai)… although whether this is the Taoist alchemical text (kei) or the Zen Buddhist poem (kai) is not clear. Another is one of the Buddhist manuscripts (a treatise on the Vimalakirti Sutra?) from the Dunhuang manuscripts (『敦煌偏文集』 = Tonkouhen bunshuu).