Green’s Dictionary of Slang

(US black) the buttocks, the posterior.

1898		C.H. Phillips Hist. of the Colored Methodist Episcopal Church in America (1925) 450: It was impossible for her to sit on the ‘do nothing stool.’ Wherever she went she just had to do something that brought the people into organized activity.
1974		E.M. Gilmer How to Win and Hold a Husband 58: [She] thought that marriage entitled a woman to sit down on the do-nothing stool for the balance of her life .
1980		J.L. Gwaltney Drylongso 16: Way back there in the slavery days [...] there was some folks that would kick this Tom dead in his do-nothing stool. [Ibid.] 168: She would rather sit on her do-nothing-stool and take all that what-I-might-say.
2001		Shields & Anderson SDropped Threads 27: To many of them think a wedding ring gives them the right to flop down on the do-nothing stool, get fat and eat onions.
2010		L. Woodruff-Brown How to Clear Your Path to Peace and Happiness n.p.: Because we are scared and will not get up off our do-nothing-stool and do what we are given the gift to do.