imminent spanking
When a character is about to be spanked, but the spanking hasn't happened yet. Typically seen when the character is in position to be spanked and the person spanking them is prepared as well with a raised hand or implement in hand. There may also be motion lines present to indicate the spanking is starting.
The ass should not have slap marks or be red, this indicates that the spanking has yet to occur. If the ass is already red, then the spanked tag is more appropriate.
For individual characters in the same illustration, spanked and imminent spanking are mutually exclusive unless its a comic or multiple views.