The Google Slap normally refers to an Adwords account. It is when your quality score goes down to 1/10, usually because of breaching policies. A QS of 1 effectively stops your ads from showing.
When the Google determines the page that visitor visit through clicking on an ad is not related, is of poor quality or download too slowly then Google takes an action called Google Slap. It drives up the amount of pay per click advertising and reduce the page rank. You can avoid Google slap by increasing Click Through Rates (CTR), by keeping landing pages relevant, by improving loading time and by providing useful information.