One of the mysteries of the English language finally explained.
In humans: a long muscle of the thigh, originating from the ilium and acetabulum and inserting into the patellar tendon, forming part of the quadriceps femoris and serving to extend the leg. Also: a corresponding muscle in other vertebrates. More fully "rectus femoris muscle".
Late 17th century. From post-classical Latin rectus + classical Latin femoris, genitive singular of femur.
rectus femoris/ˈrɛktəs ˈfɛm(ə)rɪs/
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