Chapter VI Sloka 34

c<cl< ih mn> k:[ maiw blvd Fm!,
tSyah< inh< mNye vayaeirv su:krm!.

cacala hi mana ka 
pramthi balavad dham | 
tasyha nigraha manye 
vyoriva sudukaram ( 6|34)||

The mind verily is restless, turbulent, strong and unyielding, O Krishna: I deem it as difficult to control as it is to control the wind.

Commentary: The mind constantly changes its objects and so it is ever restless.
'Krishna' is derived from Krish which means 'to scrape'. He scrapes all the sins, evils, and the causes of evil from the hearts of His devotees. Therefore He is called Krishna.
The mind is not only restless but also turbulent or impetuous, strong and obstinate. It produces violent agitation in the body and the senses. The mind is drawn by the objects in all directions. It works always in conjunction with the five senses. It is drawn by them to the five kinds of objects. Therefore it is ever restless. It enjoys the five kinds of sense-objects with the help of these senses and the body. Therefore it makes them subject to external influences. It is even more difficult to control it than to control the wind. The mind is born of Vayutanmatra (wind root-element). That is the reason why it is as restless as the wind.