The Deplorable Word was a spell from the now-dead World of Charn. If spoken with the proper ceremonies, this word would destroy all living things in the world, except for the person who spoke it. This 'ultimate weapon' was used by Jadis, the last Queen of Charn, when she lost a civil war against her sister, which destroyed the remaining population of Charn and turned the world into a sterile wasteland.
The existence of the Deplorable Word was a closely guarded secret, passed down through generations of the ruling family of Charn, and who had bound themselves with oaths to never even seek knowledge of it. Jadis, however, was far more power-hungry than her ancestors, and "learned it in a secret place and paid a terrible price to learn it".
The war with her sister, an extremely bloody world war, was apparently just a power struggle between the two, which the sister won. Jadis waited to use the word until she met with her victorious sister at the steps to her palace, offering her the chance to surrender if she acknowledged her as Charn's one true ruler. Her sister, however, refused, and so Jadis spoke the Deplorable Word, and a moment later she was the only living thing left under her world's red sun.
Afterward, Jadis showed no remorse, and even narcissistically blamed her sister for her act, declaring "Her greed has destroyed the whole world!" Besides killing Aslan on the Stone Table, this was Jadis's most evil known act.
While it is not ultimately known why Jadis didn't use the Deplorable Word to destroy Narnia when she was defeated at the Battle of Beruna, before she was killed by Aslan, there is evidence that the spell, along with numerous other magical powers she once possessed, may have only worked in the world of Charn: During her brief time on Earth, an attempt of hers to attack Letitia Ketterley with Charn magic resulted in nothing happening.
Alternatively, even if the spell were to work, she may not have had time to use it before she was killed by Aslan, since his attack was a surprise, and the use of the word required certain ceremonies. Or it may be that Aslan had already taken other unknown actions to prevent her from using it.
The Deplorable Word should not be confused with a lesser spell, also involving a magic word, which Jadis used to destroy the gates of the palace of Charn.
- It should be noted that when Polly and Digory first arrived in Charn, there were no remains of people, animals, or even plants, except for some type of dead climbing plant that had wound around a pillar. Since the Deplorable Word only affects living things, and if the vines were already dead before the spell was cast, then it explains their presence.
- Aslan warned Digory and Polly that in our own world, a secret alike to that of the Deplorable Word might soon be revealed. Given that this all took place around the year 1900, he may possibly have been referring to the atomic bomb.