A more thorough explanation:

FOY

foy (foy or fwah). [Law French] Faith; allegiance.

Foy is a term used in Law French, which means faith or allegiance. It refers to the loyalty and commitment that a person has towards a particular cause or person.

1. The soldiers pledged their foy to the king and promised to defend him against all enemies.

2. The witness swore to tell the truth, the whole truth, and nothing but the truth, so help him God, as a sign of his foy to the court.

These examples illustrate how foy is used to describe the loyalty and commitment that a person has towards a particular cause or person. In the first example, the soldiers pledged their foy to the king, which means they promised to be loyal and committed to him. In the second example, the witness swore to tell the truth as a sign of his foy to the court, which means he promised to be truthful and loyal to the court.