The concept of "Ku" is associated with the phrase "Black Magic"; that is, magic whose purpose is to injure someone.

The term "Ku" refers to certain particular methods of black magic. The method is to place poisonous snakes and insects together in a vessel until there is but one survivor, which is called the ku. The poison secured from this ku is administered to the victim, who becomes sick and dies. The ideas associated with ku vary, but the ku is generally regarded as a spirit, which secures the wealth of the victim for the sorcerer.

The word "ku" is at least as ancient as the Chinese script itself. The earliest reliable specimens of Chinese writing are inscriptions on the shells of tortoises and on the shoulder-blades of cattle, found in a Yin-Shang site at An-yang, Honan, in 1899 CE.

The Shuo wen, a dictionary from circa 100 CE., says, 

"Ku is worms in the belly." "Ku is worms in the belly." 

The commentary on the Spring and Autumn Annals (the Tso chuan) says,

'Vessel and worms make ku, caused by licentiousness. Those who have died violent deaths are also considered to be ku.

In the Pre-Han texts, the word is used in five different ways:

It indicates: (1) a disease, (2) evil spirits, (3) to cause doubt, or a woman inveigling a man, (4) a worm-eaten vessel, and grain which moulders and is blown away, and (5) a divination symbol.

The fundamental idea of ku as a disease is based on an analogy.

The human body is regarded as a vessel, into which the disease spirits enter like insects. Many ancient folks have regarded disease as due to the possession of the body by an alien spirit.

Ancient Chinese ideas associated the wind with the generation of worms. This is applied to mouldering grain, either in the sense that the chaff is blown away by the wind, or that worms generate in the grain, become insects and fly away.

It appears that the essential idea behind these meanings of ku is a loss of soul.

In the "Shih chi feng ch'an hu" it is said that "Duke Teh of Chin" instituted the fou sacrifice, killing dogs at the four gates ofthe city to dispel the ku plague." The Ch'in pen chi says, "In the second year (of Duke Teh) dogs were killed to ward off ku."

Dogs have frequently been used in Chinese apotropaic practices, from ancient times until the present.

The Book of Changes, an ancient work on divination, consisting of the explanations of sixty-four hexagrams, or figures secured in divination. 

The eighteenth hexagram is formed by the ken trigram placed above the sun trigram. The ken trigram is a symbol of mountains, of resting and stopping, and of the youngest son.

The sun trigram symbolizes wind or wood, flexibility, penetration, and oldest daughter. 

The entire hexagram is called ku.                      

According to the Book of Changes, 

"Ku indicates great progress and success. There will be advantage in crossing the great river.". . . This means that when a man divined, and secured the hexagram ku, the omen was auspicious."

It meant that the one who divined would be successful, while his enemies would be injured. Crossing the river was equivalent to an offensive military expedition.

In the case of the trigram there is very good evidence for the association of insects and wind, Huai-nan Tzu says:

"Heaven is one. Earth is two. Man is three. Three times three is nine. Two times nine is eighteen. The number eight stands for wind. Wind represents worms. Therefore worms are transformed in eight days." 

It will be noticed that the number eighteen is the number of the hexagram ku.

The Shuo en, in defining the character feng (wind), says, 

"When the wind blows, worms generate. Therefore worms are transformed in eight days."

Hsu Hao, says, 

"The wind has no form that can be pictured, so the character is made from the thing which the wind generates. Therefore the radical 'worm ' is the base of the character "wind." When the geomancer is searching for a favorable spot in the country, he observes where the wind goes, and he knows that below that spot there are ants. This is the verification of the expression, 'The wind blows, and worms generate.

This idea is clearly indicated by the written form of ku on the oracle bones.

And as ku was a kind of black magic, the hypothesis explains why the hexagram indicated success to the diviner and injury to his opponent. That was the purpose of black magic.