Construction is fully systematic:
平方 (píngfāng, 'squared') +
英寸 (yīngcùn, 'inch'). Same template gives
平方英尺 (square foot),
平方英里 (square mile),
平方厘米 (square centimeter). The
英 in
英寸 marks it as the British/imperial unit, distinct from
市寸 (Chinese inch).
