Hypocoristic

hypocoristic [hī-pə-kə-RIS-tĭk]
adj. endearing, as a pet name, diminutive, or euphemism

[From Greek hypokoristikós ("to call by endearing names) from hypo- ("under") + kor- ("child") + -istikos]

A hypocoristic name is a pet name or a diminutive name. Hypocorism is the noun form. The word hypocoristic is used in linguistics, often to explain the origin or evolution of one name into another.

Random House gives some examples:

Hypocoristic forms are, as the definition says, generally either pet names, or nicknames, such as Harry for Henry or Betsy or Beth or Liz or about a zillion others for Elizabeth, or they are forms with some sort of diminutive element, such as the suffix -y/-ie, yielding such words as preppy (prep (school) + -y), kiddie, birdie, cutie, and the like.

Ancestry-dot-com explains that the -cock suffix in names like Hitchcock and Hancock came hypocoristically:

(The name "Cocke") applied to a young lad who strutted proudly like a cock, it soon became a generic term for a youth and was attached with hypocoristic force to the short forms of many medieval personal names (e.g. Alcock, Hancock, Hiscock, Mycock). The nickname may also have referred to a natural leader, or an early riser, or a lusty or aggressive individual.

Emperor Constans II (630-668) came to the Byzantine throne in 641, 305 years after Constantine the Great, the man who Christianized the Roman Empire. In his classic tome The Decline and Fall of the Roman Empire, Edward Gibbon notes that although Constans II is known to history by that name, Constans was a pet name for Constantine:

The baptismal name of this emperor was Heraclius; he was renamed Constantine at his coronation, perhaps because his step uncle Heraclius had brought discredit on the name. He is Constantine on his coins and is so called by Nicephorus, but Tbeophanes calls him Constans and he is always known as Constans II. We must infer that Constantine was his official name, but that he was popularly called Constans in a hypocoristic sense.

Look at the map of the Byzantine Empire above and note the size of the Arab Caliphate in 650. Muhammed had been dead only 18 years, yet his religion, Islam, and his language, Arabic, were then growing like wildfire, south of Byzantium. Eventually, the Islamic Caliphate (under Oriental Turks) would entirely swallow the Christian-Greek empire north of Arabia.