It doesn’t mean Unable.
How did someone earn one ? The usual method was to fall victim to poisoning, a spell, or particularly nasty supernatural abilities. Venoms and creatures’ innate poisons dealt us a fair share of attribute damage, as did Undead. Our DM didn’t target attributes with spells very often. Or, at least, not as often as I did in my own campaign.
How frequently were they awarded ? Not often, simply because our characters’ attributes were so high that it took a lot of effort on our enemies’ part to drive any one attribute below 5. There was at least one card awarded for this ‘accomplishment’.
Any up-side to the situation ? I had fun looking up the equivalent creatures to a player’s current attribute score (Player’s Handbook, pages 8-10). “Sorry, dude, with your Strength dropped to 6 by that Large Scorpion venom, you’re now as strong as a hawk, a cockatrice, or a pixie. But you’re nowhere near as adorable as a pixie.”
The “Cards” series of posts recalls the special “trophy” business-cards awarded to players during E.G.’s original D&D v.3.5 game several years ago. This is a weekly post.