side of mouth
咡 refers specifically to the side of the mouth, the area where the lips meet the cheek.
咡 combines 口 (mouth) and 耳 (ear), likely indicating the area beside the mouth near the ear, i.e., the side of the mouth. The exact historical development is uncertain.
The mouth 口 is next to the ear 耳, pointing to the side of the mouth where the two are close.
For ジ, imagine the letter G drawn on the side of your mouth: G -> ジ, marking the spot.