Mostly from Richard Sears' Chinese Etymology site (authorisation), which in turn draws data from various collections of ancient forms of Chinese characters, including:
Shuowen Jiezi (small seal),
Jinwen Bian (bronze inscriptions),
Liushutong (Liushutong characters) and
Yinxu Jiaguwen Bian (oracle bone script).
According to Shuowen Jiezi, pictogram composed by three lines that represent a territory outside the city walls and its nearby villages and greenfields, a territory that belongs to the incontaminated countryside, distant from every kind of civilization.
As an alternative, it's the original form of 堂, hence it's the profile of a building. The reversed character is 凵.