Auxilium ad filium militem faciendum et filiam maritandam was an ancient writ directed to the sheriff of every county, 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.

References

1. 
2. English, Arthur. Dictionary of Words and Phrases Used in Ancient and Modern Law. ISBN 978-0-8377-2104-0.