Frombe- +‎swear.

beswear (third-person singular simple presentbeswears,present participlebeswearing,simple pastbeswore,past participlebesworn)

1. (transitive) Toswear by; swear about or regarding; swear at or to;promise
   • 1888, Teutonic mythology, Jacob Grimm, James Steven Stallybrass, tr. by J.S. Stallybrass - Volume 4:

     Some masters of the art take a pure child and set him in their lap, then lift his hand up and let him look into his nail, andbeswear the child and the nail with a great adjuration, and then speak in the child's ear three unknown words, whereof one is Oriel, the others I withhold for fear of offending.

   • 1895, The Saga Library:

     Thereupon they met themselves, and then peace was talked over, and this was the matter agreed on, that the kingsbeswore each other brotherhood and settled peace between them while they should both be alive; [...]

   • 1903,The Menorah, volume34:

     Abrahambeswore his servant, Eliezer, by putting the latter's hand under his thigh.

   • 2013, Shaul Shaked, Siam Bhayro, James Nathan Ford,Aramaic Bowl Spells:

     Ibeswear you that you should be struck in your pericardium by the lance of the mighty [Qitaros], who is ruler over demons,dēvs and liliths.

   • 2016, Bucky Williams—Hooker PhD,Angel Wing Begonias:

     But if you willbeswear what / Others think and be my love [...]

• bewares