Mjölnir (pronounced MIOL-neer) is well known as the hammer of Thor (Norse god of thunder) - but there is so much more history & mythology behind it. Let's unpack it below.
Mjölnir is written different ways depending on which culture or set of languages you are referring to. Other common variations are Mjølner (Danish) & Mjölner (Swedish). For the purpose of this article, we will be using the modern Icelandic variant of Mjölnir. The precise etymology is unknown, however, many scholars agree it stems from an Indo-European root meaning lightning/thunder. Many other meanings have been discussed such as snow or white linking to the colour of lightning or representing purity.
A popular collection of Norse literature known as the Edda written approx. 1200AD - 1250AD discusses the creation of the hammer being made by two dwarven brothers Brokkr & Eitri - known as the greatest smiths in all the 9 worlds. Brokkr & Eitri had not intended the handle of the hammer to be so short - leaving it ineffective as a blacksmith tool and as a weapon. The dwarves made the hammer as part of an elaborate bet between Thor & the trickster god Loki (pronounced LOAK-ee). Loki had stolen a hair from Thor's wife, Sif, leading Thor wanting to kill Loki. Loki was granted prolonged life as he had promised to obtain a full head of hair even more marvellous than the current set. Whilst on the hunt for this, he had cunningly pitted many dwarven smiths against each other to prove who was the greatest smith in all of Svartalfheim (a subterranean complex of mines and forges - home of the dwarves). Loki had returned with not only Mjölnir but other treasures & offerings for the other gods to get back into good esteem.
The hammer was worn with great meaning in 9th - 11th century Scandinavia, with many who believe it was worn as a defiance to the ever growing Christian influence of the time. You may note similarities between the Mjölnir and the Christian Crucifix. Others wore it as a symbol of blessing, consecration & protection. The symbol remains today and is worn typically as a means to honour the Norse god Thor, and to remind them of attributes Thor is known for. The 2 most common attributes Thor represents are strength and fertility. The symbol reminds the wearer to show strength particularly in adversity, and that all endeavours ultimately should be linked back to the creation or protection of his or her family. The hammer was also used a ceremonial tool, used in blessings at births, wedding & funerals.