When two lesbian chickies have hardcore vagina-to-vagina sex.
"Omg, did you hear about Kelsey and Heather clam slapping last night I heard it was hot"
An activity that lesbians and curious young women do to each other to relief stress. Involves two females pressing their "clams" against each other and making thrusting motions to maximize their pleasure.
My lesbian friend loves clam slapping with her girldfriend.