Plunged is a past tense term meaning fucked/had sex/slept with
Hand-me-down in the context of talking about a girl or guy in which a friend or colleague has/wants/is with: the term is used when someone has been plunged by you and your friend is now plunging her/him.
The term shares a similar meaning to sloppy-seconds.
Person 1: I had sex with person 2 last week
Person 3: you had sex with my hand-me-down