Said of a female, "sloshing in their boots" or "sloshing in her boots" refers to a state of sexual attraction or arousal so extreme that they're "wet" (i.e., vaginally lubricated) all the way down to their feet.
"Did you see that selfie of Angela Merkel with that handsome stud Syrian migrant? The old bag was sloshing in her boots!"
by Honeyko September 19, 2015