Antonyms of feeble:
strong, powerful, robust, bright, clear, daring, fresh, resolute, sturdy, brilliant, conspicuous, energetic, hearty, vigorous, able-bodied, athletic, beefy, brawny, fit, husky, muscular, sinewy, strapping, virile, hard, hardy, lusty, red-blooded, tough, fortified, hardened, inured, strengthened, toughened, energized, invigorated, vitalized, hale, healthy, sound, capable, competent, convalescing, recovering, recuperating, mighty, rugged, stalwart, stout.
Examples of usage:
  1. " My wife is still slightly feeble," he said.
    "Vittoria, v8", George Meredith.
  2. But you're feeble as a kid baby.
    "Smoke Bellew", Jack London.
  3. But he thought that it behoved him to do this work; and therefore, old and feeble as he was, he set himself to his task.
    "Orley Farm", Anthony Trollope.