dumb
瘂 centers on the inability to speak: being mute or dumb. It describes a physical condition of voicelessness.
瘂 combines the sickness radical 疒 with 亞, which likely provides the sound. The character denotes the medical condition of muteness.
The sickness enclosure 疒 wraps around 亞, a shape that looks like a blocked throat or closed mouth. Picture an illness that seals the voice inside, leaving a person unable to speak.
For ア, imagine a person with muteness trying to say 'ah' but no sound comes out: the silent 'ah' cues ア while the sickness enclosure shows the reason.