an exclamation of confirmation
誒 is a rare kanji used for an exclamation of confirmation, similar to an interjection like "yes" or "indeed". Its standalone meaning is limited to this expressive use.
誒 combines 言 (speech) with 矣, which likely contributes a phonetic element. The exact historical development is uncertain, but the structure suggests a character related to spoken exclamations.
The left side 言 represents words or speech, and the right side 矣 looks like a person with an open mouth. Imagine someone nodding and saying "yes" with a confirming tone.
For キ, picture a person giving a quick confirming "key" word to unlock a conversation: key -> キ, and the speech radical 言 reinforces the spoken confirmation.