A tampon. Use of this slang for "tampon" was made popular in the 1976 horror movie, "Carrie," starring Sissy Specek and based on a Stephen King novel. In the movie there is a scene where Carrie, showering after gym class at school, begins to menstruate for the first time. She is frightened because her extremely religious mother had never told her about periods. Carrie, no stranger to classmate bullying, is teased about this and the girls in the shower chant, "plug it up, plug it up, plug it up" while throwing tampons at her.
Girl to friend: Let's go swimming.
Friend: I can't. I just started my period.
Girl to Friend: So? Plug it up and let's hit the beach!