When one is having relations of the sexual nature on the beach. The man upon nearly cuming (perferably after a substantial time after the comencing of intercorse). withdraws from the female luv hole (aka box) and finishes himself off, squirting his sticky man juice all over the girls box. then with the opposite hand grabs a handfull of sand and chucks it at the girls box causing it to stick. Hence the name sand box (not to be confused with the common childhood play area)
I just gave that girl a sand box. That's why see walks like she's got sand paper between her leg.