belch, hiccup
呃 centers on involuntary mouth sounds: a belch or a hiccup. Both meanings share the idea of a sudden, uncontrolled release of air from the mouth.
呃 combines 口 (mouth) with 厄, which likely contributes the sound and may relate to a sudden or difficult action. The exact historical development is uncertain.
The mouth 口 is beside 厄, which looks like a person bending over. Imagine someone bending over and making a sudden belch or hiccup from their mouth.
For アク, picture a sudden hiccup that sounds like 'ack!' — ack -> アク, and the mouth 口 releases the sound.