Byzantine emperor Constantine V (ad 718-775) was known as Kopronymous (dung-named) by his enemies. According to legend, when he was baptised, he lost control of his bowels into the baptismal font.
Louis XI of France (1423-83) was called the Universal Spider because his plots and conspiracies webbed across Europe. He was friends with Philip the Good to whom he gave money for crusading, and enemies with Philip’s son Charles the Bold who felt his father was wasting his inheritance chasing after heretics.
Eystein the Fart’s (1088-1123) epithet comes from the Norwegian word fart, which means ‘fast’, due to his extensive travelling. He was also one of the first people to describe ice-skating: he boasted of his skills on ‘ice-legs’. He was succeeded by his son, Halfdan the Mild.
King Ragnar Hairy Breeches (Ragnarr Loðbrók in Norse) was a legendary figure who, according to the Sagas, was briefly King of Denmark. He got his name from the animal skin trousers made by his wife which supposedly protected him in battle.
Louis XVIII of France (1755-1824) was known as The Unavoidable (l'Inévitable) as, when he was placed on the French throne after the deposition of Napoleon, there wasn’t really anyone else available.
John George I, Elector of Saxony (1585-1656) was nicknamed ‘The Beer Jug’ for his love of alcohol. He was also a lover of hunting, and was involved in the killing of 113,629 wild animals.
A nickname is the hardest stone that the devil can throw at a man.
Alfonso VI of León lost Seville in a game of chess. In 1078, during the siege of Seville he agreed to play chess against Ibn Ammar, who was the close friend of the ruler of the Moorish city-state, Muhammad Ibn Abbad Al Mutamid. Ibn Ammar brought his own lavish board with him for the match and suggested a wager to make the game a shade more interesting. If Alfonso won he could keep the valuable board and pieces, but if he won then the king had to grant him any wish.
The two men sat down to play and Alfonso quickly realised that Ibn Ammar was one of the greatest players of his age. True to form, Ibn Ammar won the game and Alfonso, in some trepidation, asked what he would demand. To this Ibn Ammar, putting duty before personal gain said he wished the king to turn away from Seville and spare the city. Alfonso, being a gentlemen, did exactly that.
Sancho I (died ad966) was another king of León. He was known as Sancho the Fat (for obvious reasons). After ruling for two years, he was deposed by the nobles led by the Fernán González of Castile, because of his extreme obesity. While in exile, he managed to lose ‘at least some of his girth’ and managed to reclaim the throne two years later.
Alfonso IX of León was known as Alfonso the Slobberer due to his habit of foaming at the mouth when angry, and Bermudo II of León was known as 'Bermudo the Gouty'.
Henry the Impotent of Leon was supposedly impotent as the result of a curse by the Bishop of Toledo. The curse was called frigidi et maleficiati and was fairly common in medieval times; though it didn't seem to work, as Henry had a daughter. His enemies decided, though, that he must've been cuckolded (as he may well have been) and that the daughter belonged to Beltrán de La Cueva, 1st Duke of Albuquerque.
If you don't want the nickname, don't live up to it.
In his own time, King John was known disparagingly as ‘Softsword’ for his shambolic military adventures.
Richard the Lionheart's brother, John, was nicknamed 'Dollheart.'
Charles the Silly of France was married to Isabella the Great Sow.
Edward III was known as ‘Edward the Bankrupt’, though he preferred to be known as ‘King of the Sea’.
Ibrahim the Mad (1615-48) was Sultan of the Ottoman Empire. Until he was 23 years old, he was locked up in a windowless building by his brother Murad IV. As a result Ibrahim acquired some odd sexual proclivities. He liked the larger lady, and once saw the private parts of a cow and ‘sent the shape of them in gold all over the Empire with orders to make enquiries whether a woman made in just that manner could be found for his lust’. They found a suitable candidate in Armenia, weighing 150kg, and she entered the harem under the name Sechir Para (Sugar Cube). Ibrahim became besotted with her and made her Governor General of Damascus.
In Russia, Иван Грозный (Ivan Grozny) doesn't mean 'Ivan the Terrible' in the way that we might think. It is slightly less negative than in English, something like Ivan the Fearfully Great. However he was definitely terrible in the other sense; he became Tsar after feeding the head of the government to a pack of dogs and killed his son in a rage, after the two got into an argument over Ivan’s daughter-in-law wearing immodest clothes.