Joking euphemism for having been sexually groped without one's consent.
Based on the name of a prominent, once-thought-to-be heterosexual (???), Republican spokesperson and organizer who allegedly groped his chauffeur repeatedly and persistently—without consent. The driver claimed the experience traumatized him and complained about it to anyone who would listen, including the Press.
(Straight guy, realizing he's just been pinched on the ass by another man) "Egads! I've just been Schlapped!"