String slap is an archery term that refers to the event when a bow string strikes the inside of the forearm when a bowstring is released.
They degree of string slap dependent on a number of factors including the distance the string is from the bow itself and the position of the arm holding the bow.
A device called a bracer can be worn to protect against the impact. Medieval bracers were of horn or leather construction. There does appear to be a level of 'tough than thou' with smaller bracers being preferred.