The ‘monkey mind’ or ‘monkey-like mind’ (kapicitta) is a term occasionally used by the Buddha to describe the agitated, easily distracted and incessantly moving behaviour of ordinary human consciousness (see for example Ja.III,148; V,445).
Once he observed: "Just as a monkey swinging through the trees grabs one branch and lets it go only to seize another, so too that which is called thought, mind or consciousness arises and disappears continually both day and night" (S.II,93). Anyone who has spent even a little time observing their own mind and then watched a troop of monkeys will have to admit that this comparison is an accurate and not very flattering one.
On another occasion the Buddha said that a person with uncontrolled craving "jumps from here to there like a monkey searching for fruit in the forest" (Dhp.334). In contrast to this the Buddha asked his disciples to train themselves so as to develop "a mind like a forest deer" (miga bhutesu cetasa, M.I,450). Deer are particularly gentle creatures and always remain alert and aware no matter what they are doing.