The origin of 勾 and 股 in The Pythagorean Theorem 勾股定理
in ancient china, the right angle triangle is named "勾股形".

the 國語辭典 has this entry: 勾股, explained as


  我國古代稱直角三角形為「勾股形」。其直角旁的短邊稱為「勾」，長邊稱為「股」，對著直角的邊稱為「弦」。


the short side near the right angle is called "勾", the longer side near the right angle is called "股"; and the side opposite the right angle is called "弦".

people used something like "勾股", "勾股之法" to describe the equation a²+b²=c²; nowadays, a hybrid naming is used "勾股" (the ancient chinese name) + "定理" (translation of theorem).

btw, the character 句 (u+53e5) and 勾 (u+52fe) mutually borrowed in ancient time.