There are different versions as to the origin of 亚, among which is the version that 亚 is a building with four sides. 亚 serves as phonetic component to form characters.
In 说文解字, 亚 is a humpback ( 醜也。象人局背之形).
In modern Chinese, 亚 means：second, inferior, sub-
Characters formed by the phonetic component 亚
哑 痖 yǎ
垭 桠 yā
铔 娅 氩 挜 yà