I've never heared of this before.
The Overton Window refers to a strategy -- used by politicians to influence public response to policy proposals, but almost certainly useful to most of us -- where a clearly unacceptable option is presented to the public first, with the hope that a second option would be seen as being more acceptable, and more acceptable had that second option been presented first.
Suppose in order to balance the budget, taxes have to be raised 5%. The public will not want that to happen. But suppose that the the first proposal presented is a 15% tax increase. Opposition would be heated, but as a compromise, a 5% increase will be accepted because it seems palatable by comparison.
Nate Silver argues that this might be happening in the names Obama is floating for potential Supreme Court picks.
