1. a. To decide or declare that one will not or will no longer engage in (an activity or habit, for example): a political group that has forsworn violence.
b. To decide or declare that one will not or will no longer use or be associated with (something): "He had cast in his lot with the lions and forsworn the lambs" (Robert Louis Stevenson).
c. To disavow under oath: "He was forced to take an oath forswearing heretical views" (Garry Wills).