What is the X in X-mas?
early 15c., "assertion that something is true," from Old French afermacion (14c.), from Latin affirmationem (nominative affirmatio) "an affirmation, solid assurance," noun of action from past participle stem of affirmare (see affirm). In law, as the Quaker alternative to oath-taking, it is attested from 1690s.
in law, a promise by a witness concerning testimony allowed in place of an oath to those who cannot, because of conscience, swear an oath. For example, members of the Society of Friends (Quakers), Jehovah's Witnesses, and other persons who have objections against taking an oath are allowed to make affirmation in any manner they may declare to be binding upon their consciences in confirmation of the truth of their testimony.