In Norse mythology, Mimir is a giant. He was the wisest of all and became a hostage of the Vanir after the war with the Aesir. He was initially beheaded by the Vanir and his head was returned to them. It stands embalmed at one of the springs under Yggdrasil, known as Mimir's well, and was given Odin's eye as a pledge of his wisdom.
Mímir is said to have been the supposed son of the giant Boltorn, which would make him Bestla's brother and thus Odin's maternal uncle, in the Gylfaginning it is stated that as the keeper of the spring Mímir drank its water with the Gjallarhorn, a drinking horn that shares its name with the one Heimdallr will use to announce the beginning of Ragnarǫk, the end of the world.
For the interpretation of the name Mímir, in Old Norse Mímir, scholars are split into two sides:
According to some, the name Mímir is closely associated with wisdom and memory, and would mean "one who remembers". The name would thus be related to the Old English mimorian, the Latin memor, and would come from the Indo-European root *smer-, *mer-, "to remember".
For other researchers like François Xavier Dillmann, the name would be close to the Norwegian meima "measure", and would derive from the Indo-European *mer- "to measure" with the meaning of "the one who measures the destiny".
Mimir was the Aesir god of Wisdom and one of the two deities (with Hœnir the Undecided) sent to the Vanes to promote peace, in exchange for Njord, Freyr and Freya. But the latter, realizing that they had been fooled, beheaded the god and sent his head to the Aesir.
However, Odin coated it with a mixture of herbs so that it would not rot and he enchanted it with spells. Once brought back to life, he was able to speak and reveal hidden secrets, many truths that no one else knows.
Odin placed him under the roots of Yggdrasil, he thus became the guardian of the Mimisbrunn (the "spring of Mímir"), the spring that contains wisdom and intelligence, in Jötunheim under one of the roots of Yggdrasil.
One day, when Odin had just created the world, he came to Mímir in order to drink from his spring to gain great wisdom. Mímir agreed, but in exchange, Odin had to give him one of his eyes, which he did. Thus, Odin became one of the wisest gods.