What is Auxilium Ad Filium Militem Faciendum Et Filiam Maritandam?

Legal Definition
Auxilium ad filium militem faciendum et filiam maritandam was an ancient writ directed to the sheriff of every county in England, where the king or other lord had any tenants, to levy of them reasonable aid, towards the knighting of his son, and the marriage of his eldest daughter.

The writ was also known as Auxilium ad filium primogenitum militem faciendum, vel ad filiam primogenitam maritandam.
-- Wikipedia
Legal Definition
An ancient writ which was addressed to the sheriff to levy compulsorily an aid towards the knighting of a son and the marrying of a daughter of the tenants in capite of the crown.
-- Black's Law Dictionary
Legal Definition
An ancient writ addressed to the sheriff to levy compulsorily an aid toward knighting of a son and the marrying of a daughter of the tenants in capite of the crown.—Black.
-- Ballentine's Law Dictionary