Resistant or impervious to balls, either in the sense of withstanding impacts from projectiles or protected from ball-shaped objects.
From 'ball' plus 'proof,' using the English suffix meaning 'resistant to' or 'protected from,' similar to waterproof or fireproof.
While 'ballproof' is rarely used, it's a logical English construction—we can imagine medieval armor makers or tank designers discussing how to make something 'ballproof' against cannon fire, yet today it's almost disappeared from English.
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