Forsworn means to have broken a vow (in Juliet's case, her marriage vows).
"Ancient damnation! O most wicked fiend! / Is it more sin to wish me thus forsworn, / Or to dispraise my lord with that same tongue / Which she hath praised him with above compare / So many thousand times?"
Juliet, from Shakespeare's Romeo & Juliet; Act III, Scene 5.