rump; butt flesh
Colloquial term for the flesh of the buttocks, often used in casual or humorous contexts.
彼は椅子から立ち上がるとき、尻肉が痛いと言った。
When he stood up from the chair, he said his butt flesh hurt.
Compound of 尻 (buttocks) and 肉 (flesh, meat). The reading is a straightforward kun'yomi combination.