expression, noun
lazy ass; slow to act; reluctant to get moving
Idiomatic expression literally meaning 'heavy buttocks'. Used to describe someone who is sluggish, unwilling to get up, or slow to take action. Often used in a teasing or mildly critical way. Compare with 腰が重い, which focuses more on reluctance to start a task.
See also: 尻が重い
彼は重い尻で、なかなか動こうとしない。
He's a lazy ass and won't get moving easily.
会議が終わっても重い尻を上げない人が多い。
Even after the meeting ends, many people are slow to get up.