for·swear also fore·swear 
(fôr-swâr)
(fôr-swâr)v. for·swore (-swôr), for·sworn (-swôrn), for·swear·ing, for·swears
v.tr.
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).
2. To make (oneself) guilty of perjury.
v.intr.
Idiom: To swear falsely; commit perjury
be forsworn
To commit perjury.
[Middle English forsweren, from Old English forswerian : for-, wrongly; see FOR- + swerian, to swear; see SWEAR.]