Time is an eraser rather than a builder, and a good one at breaking the fragile—whether buildings or ideas.
fragility was simply vulnerability to the volatility of the things that affect it
For the fragile, shocks bring higher harm as their intensity increases (up to a certain level).
Your car is fragile. If you drive it into the wall at 50 miles per hour, it would cause more damage than if you drove it into the same wall ten times at 5 mph. The harm at 50 mph is more than ten times the harm at 5 mph.
Jumping from a height of thirty feet (ten meters) brings more than ten times the harm of jumping from a height of three feet (one meter)—actually, thirty feet seems to be the cutoff point for death from free fall.
For the fragile, the cumulative effect of small shocks is smaller than the single effect of an equivalent single large shock.
For the antifragile, shocks bring more benefits (equivalently, less harm) as their intensity increases (up to a point).
Simply, if for a given variation you have more upside than downside and you draw the curve, it will be convex; the opposite for the concave.
If you double the exposure to something, do you more than double the harm it will cause? If so, then this is a situation of fragility. Otherwise, you are robust.