Sökmímir or Søkkmímir was a jotun who appears in two sources from Norse mythology, suggesting that he was once a well-known giant in Scandinavia.
In Grímnismál, stanza 50, it appears that Odin killed the giant:
He notably appears in Ynglingatal, where subterranean abodes of giants are called Sökkmímir's halls:
References
Sökkmímir Wikipedia(Text) CC BY-SA