Ischium comes from iskhion, the Greek word for the socket which receives the head of the femur (now called the acetabulum). The word was sometimes used simply for the hip joint.
Galen, in the second century A.D., divided the hip bone (os coxa) into the three bones we know today: the ilium, pubis, and the iskhion, which was later Latinized to ischium. These three bones fuse into the single hip bone during puberty.
The ischium receives the weight of the body when we are sitting: Though "we sit on the ish” is a mnemonic, the proper pronunciation is "ISS-kee-um" in honor of its Greek forebearer iskhion.
The unfused ischium of a two-year-old (bottom right).
Also seen are the pubis (bottom left) and ilium.