By A.E. Waite (1911), tarot card illustrations by Pamela Colman Smith (1911)
Power, energy, action, courage, magnanimity; also complete success and honours.
Divinatory Meanings – Reversed
Despotism, abuse of power, weakness, discord, sometimes even disgrace.
A woman, over whose head there broods the same symbol of life which we have seen in the card of the Magician, is closing the jaws of a lion. The only point in which this design differs from the conventional presentations is that her beneficent fortitude has already subdued the lion, which is being led by a chain of flowers. For reasons which satisfy myself, this card has been interchanged with that of justice, which is usually numbered eight. As the variation carries nothing with it which will signify to the reader, there is no cause for explanation. Fortitude, in one of its most exalted aspects, is connected with the Divine Mystery of Union; the virtue, of course, operates in all planes, and hence draws on all in its symbolism. It connects also with innocentia inviolata, and with the strength which resides in contemplation.
These higher meanings are, however, matters of inference, and I do not suggest that they are transparent on the surface of the card. They are intimated in a concealed manner by the chain of flowers, which signifies, among many other things, the sweet yoke and the light burden of Divine Law, when it has been taken into the heart of hearts. The card has nothing to do with self-confidence in the ordinary sense, though this has been suggested–but it concerns the confidence of those whose strength is God, who have found their refuge in Him. There is one aspect in which the lion signifies the passions, and she who is called Strength is the higher nature in its liberation. It has walked upon the asp and the basilisk and has trodden down the lion and the dragon.
8. Fortitude. This is one of the cardinal virtues, of which I shall speak later. The female figure is usually represented as closing the mouth of a lion. In the earlier form which is printed by Court de Gebelin, she is obviously opening it. The first alternative is better symbolically, but either is an instance of strength in its conventional understanding, and conveys the idea of mastery. It has been said that the figure represents organic force, moral force and the principle of all force.
By S. L. MacGregor Mathers, c 1888
Strength, or Fortitude – Power, Might, Force, Strength, Fortitude.
Divinatory Meanings – Reversed
Abuse of Power, Overbearingness, Want of Fortitude.
Symbolism of the Keys
Strength or Fortitude – A woman crowned with crown and cap of maintenance, who calmly, and Without effort, closes the jaws of a furious lion. She represents Strength.
General Book of the Tarot
By A. E. Thierens, 
Description and Meaning
The astrologer says, that the Eleventh house is the house of the ‘friends.’ This means, that it contains those who are with us, and that which we have within the limit of our power, because ‘friendly,’ is that which is understood. The forces of nature, which we have mastered, are friendly to us and this is very well expressed by the woman who “is closing the jaws of a lion.” The latter stands for passion more particularly. She derives this force from the eternal or superhuman and this is indicated by the lemnescate above her head. In older editions of the card 4 we find half the symbol for Aquarius, as a line of vibration added to it: Viewed from a purely astrological standpoint it is evident, that the force to conquer Leo should be found in the opposite sign, Aquarius. Early Renaissance must have seen this in the same way, as we find exactly the same image–only with one difference: it is there a young man, not a woman–a man closing the jaws of a lion in the capital of a pillar in the church of St. Andrew-the-Less in Vienna. Which proves at the same time, that the chosen image is not of a very recent date. (Musée du Trocadéro: Paris.)
Papus identifies it with the Hebrew letter Kaph, which he says “is a reinforcement of the Gimel–(Gemini)–so that we might say that it designates the hand of man in the act of grasping strongly. Ideas of strength are therefore applied to this letter.” We should say it is the grip of friendship. A well-known symbol in many societies of brotherhood consisted of two hands united in a close grip of friendship.
“It is connected with the mystery of union . . . in all planes . . .” (Waite), and this also is evident, because we are united with that which we have mastered and with people who are able to respond to our (electric) emanations of thought, or to whose emanations we ourselves respond.
By Paul Foster Case (1933)
11, La Force
(See Lesson 1, concerning the number of this Key and its relation to the sign Leo). This card corresponds to the Hebrew letter Teth, and thus to the zodiacal sign Leo. It refers, in the magical side of occultism, to the law which is the basis of all human control of occult forces.
Ill-dignified, it may signify ostentation, obstinacy, and the like. often, in material questions, it relates to risky speculation, dangerous adventures, and sometimes to clandestine love-affairs. Much depends upon the dignity of the card in the layout.
More information on Strength Tarot card.