The capacity to bear load without failing. Its root runs through valid, value and valour — all of them mean able to hold.
Old English strengþu (strength), from strang (strong, firm), from Proto-Germanic *strangithō, itself from *strangaz (tight, drawn taut); the capacity to bear load was named, at root, for being pulled tight rather than left slack.
The capacity to bear load without failing. Long before it named a virtue, this word named a rope pulled taut rather than left slack — holding, here, is what a taut thing does.