level best, to do one's

To exert oneself to the fullest. A nineteenth-century Americanism, this term has been traced to the days of the California gold rush, when miners panning for gold would shake sand and gravel until it was level and revealed the ore. It appeared in An Arkansaw Doctor in 1851: “We put our horses out at their level best.” Mark Twain also used it, in a poem, “He Done His Level Best” (1875): “If he’d a reg’lar task to do, he never took no rest; or if ’twas off-and-on, the same, he done his level best.”