I want to ask the idiomatic English expression of washing my ass.

It's supposed to be a butt wash. Ass means ass, ass.

Hip is the posterolateral area of hip bone, which is composed of a large number of hip muscles and rich blood vessels and nerves. On the deep surface of gluteus maximus, there are two channels, namely, ischium foramen and gluteus maximus foramen, and each channel has a muscle and a group of blood vessels and nerves passing through it. The obturator internus muscle passes through the ischial foramen; The piriformis muscle passes through the ischial foramen and is divided into the piriformis superior foramen and the piriformis inferior foramen.

The foramen of piriformis is the notch on the upper edge of piriformis. The vascular nerves passing through the piriformis foramen are superior gluteal nerve and superior gluteal nerve in turn from outside to inside.

The inferior foramen of piriformis is the notch of the lower edge of piriformis. The vascular nerves passing through the piriformis muscle are sciatic nerve, posterior femoral cutaneous nerve, inferior gluteal nerve, inferior gluteal artery, inferior gluteal vein, internal pudendal artery, internal pudendal vein and pudendal nerve from outside to inside.

The ischial foramen is the space between the sacral tubercle ligament and the sacrospinous ligament. The vascular nerves passing through the ischial foramen are internal pudendal artery, internal pudendal vein and pudendal nerve from outside to inside. Obturator tendon is located in the deep layer of blood vessels and nerves.

I hope it can help you solve the problem.