Physical resources, stamina, staying power; substance, strength of character, dependability. Now colloquial and somewhat archaic.

in a race or other contest (hence also gen.), power to ‘stay’ or continue in action for a long time; power of persistent effort.

Sport. Of a horse: Condition in regard to health and training; fitness for running or racing; style and speed in running (as compared with…

Sport. Power of endurance or sustained effort.

the condition of being match-fit.