laughing
謚 centers on laughing, specifically a kind of amused or derisive laughter expressed through words.
謚 combines 言 (words, speech) with 益, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to speech.
Words (言) that overflow (益) with amusement: imagine someone's speech bubbling over with laughter.
For エキ, picture a person laughing so hard they make an 'eki eki' sound, like a stifled giggle: eki -> エキ.