Abarim Publications' online Biblical Hebrew Dictionary
The verb כתם (katam) means to stain and is used in Jeremiah 2:22 only in the phrase "the stain of your guilt" (עון, 'awon). This verb occurs in other languages as well, with similar meanings, but in Arabic, Akkadian and Assyrian it means to cover, which suggests that Jeremiah 2:22 possibly speaks of the covering of guilt rather than the stain of it. Perhaps similarly, the verb כפר (kapar) means to cover (and to atone), whereas the noun כפור (kepor) denotes a bowl of gold or silver.
Noun כתם (ketem) is one of a few words for gold, used predominantly in Biblical poetry (a more frequently used word is זהב, zahab). This particular noun כתם (ketem) appears to be not related to the highly similar verb כתם (katam) but rather a loanword from Egyptian, although its adoption into Hebrew may have been lubricated by the verb. Gold is mentioned in the Bible mostly as a metaphor for absolute truths or for knowledge that can't be corroded by anything natural, and the dynamic between the "gold" of the Hebrews and the "gold" of the Egyptians may be the same as that between the alphabet (see our article on YHWH) and hieroglyphs.