"easy victory," 1838, such as one that happens in the absence of competitors, when the solitary starter can traverse the course at a walk. Transferred sense of "anything accomplished with great ease" is attested from 1902. To walk (all) over (someone) "treat with contempt" is from 1851.
Examples from the Web for walk-over
The victory which is merely a walk-over is scarcely a victory.The Conquest of Fear
The expression is used advisedly, for it was nothing but a "walk-over."Destruction and Reconstruction:</p>
You see, he considers that the walk-over which was anticipated has degenerated into a stroll.Scarlet and Hyssop
E. F. Benson
With him out of the way it would be a walk-over for the Sox.Baseball Joe in the World Series
And no money shall be awarded to a boat for a walk-over, but she may fly a flag therefor.Yachting Vol. 2