Scack is a friendly punch to the shoulder, possibly inflicting minor pain, but not done in malice. It can also be a congratulatory hard slap to the back for something well done, but it's not just a light pat. Scack can be used as a verb or a noun.
This is an expression used to denote something that is irrelevant, useless or unfit for purpose. If you wanted to say someone was stupid, you could say “Her head is as empty as a eunuch’s ball sack.” If you wanted to say someone was useless, “He’s as much use as a eunuch’s ball sack.” Cowardly “If courage was balls you’d be a eunuch’s ball sack.” The expression is versatile and can be adapted to a wide variety of purposes.