moan
喟 centers on a low, mournful sound: a moan or sigh expressing grief or lament.
喟 combines 口 (mouth) with 胃, which likely contributes the sound. The character expresses a vocal sound of lament.
A mouth 口 beside a stomach 胃: imagine someone clutching their stomach and letting out a low moan of pain or grief.
For キ, picture a person moaning 'key...' as they search for a lost key, their voice a low lament: key -> キ.
sighing