U.S. colloq. Also thank’ee-marm. A hollow or ridge in a road, which causes persons passing over it in a vehicle to nod the head involuntarily, as if in acknowledgement of a favor; spec. a ridge or hollow on a hill road serving to throw off descending rain-water.
1849. Longf., Kavanagh, xi. We went like the wind over the hollows in the snow;—the driver called them ‘thank-you-ma’ams,’ because they made everybody bow.
1867. O. W. Holmes, Guard. Angel, xiv. Life’s a road that’s got a good many thank-you-ma’ams to go bumpin’ over, says he.
1897. Howells, Landl. Lion’s Head, 192. At one of the thank-you-marms in the road, the sick man stopped, like a weary horse, to breathe.
