Summary

• Palpatine genuinely believed Plagueis to be powerful and deserving of the title "Wise."
• That said, Plagueis underestimated Palpatine, leading to his own demise at the hands of his apprentice who sured his master's wisdom.
• Darth Sidious' betrayal marked the ing of the torch according to the Rule of Two, with Palpatine finally suring his master's power.

There's a reason why Star Wars canon. Having been referenced in Revenge of the Sith as the powerful Sith master of Darth Sidious himself, the origins of Palpatine's teacher and his legendary abilities have largely been left a mystery in the established canon, with Plagueis only recently making his first live-action debut in the finale episode of The Acolyte. However, the non-canon Legends confirmed that Plagueis received his title from Palpatine himself.

In the non-canonical Legends novel Darth Plagueis by James Luceno, the origins of Palpatine are revealed. Not only does the book uncover how Palpatine was discovered by Darth Palgueis and eventually became his new Sith apprentice, but it also reveals the years of meticulous planning and "Grand Design" the Sith engineered to make the galaxy theirs in the wake of the Jedi Order's destruction. To that end, the Star Wars novel confirms that Palpatine naming Darth Plagueis as "The Wise" was neither ironic nor simply for the sake of flattering his dark master.

Darth Plagueis' Nickname Wasn't Meant To Be Ironic

"Powerful Beyond Palpatine's Understanding"

In Revenge of the Sith, Palpatine notes during his gradual corruption of Anakin Skywalker that Plagueis' demise was ironic considering the fact that while he could save others from dying, he could not save himself. However, the Darth Plagueis novel confirms that the Sith Lord's title given to him by his apprentice was not created with any sort of inherent irony. During their conversations and planning for the future where Palpatine is envisioned as Supreme Chancellor of the Republic while Plagueis operates in the background as co-chancellor, Palpatine is truly reverent to his master: “In the annals of Sith history, you will be known as “Darth Plagueis the Wise”.

As confirmed in a later age of the novel, it's confirmed that Palpatine was indeed being genuine with the naming of his Sith master. Fearing the power and knowledge at his master's command, Plagueis' title was both a strong reminder and a warning for Palpatine himself to continue learning the ways of the dark side:

"Palpatine hadn’t been attempting to flatter Plagueis by calling him “Wise”, not entirely at any rate. The Muun was powerful beyond Palpatine’s present understanding, the only being capable of guiding the galaxy into the future. A crescendo. At times, it was difficult to grasp that they would see in their lifetime the fall of the Republic and the annihilation of the Jedi Order. And yet Palpatine seemed to know it to be true. A grand design was unfolding, in which Palpatine was not merely a player, but an architect."

However, Palpatine was content with biding during his time when he was still growing in his own dark side, all while never letting his guard down around the far more powerful Sith Lord.

As Wise As He May Have Been, Darth Plagueis Underestimated Palpatine

Darth Sidious Eventually Sured His Master

Eventually, Palpatine comes to believe the time had come in the wake of his election as Supreme Chancellor to finally kill his master, in accordance with the Sith's Rule of Two. Blindsided by the betrayal, it was evident that Darth Plagueis completely underestimated his apprentice. Drawn to the murderous act by the dark side itself, Palpatine kills his master using multiple blasts of Force lightning after Plagueis had grown intoxicated from drinking and had begun to sleep.

It's currently unknown if the same series of events seen in the Darth Plagueis novel occurred in the established Star Wars canon. After all, the book's technical status is Legends, though it was one of the last novels written prior to the continuity purge. Regardless, the suggestion remains that Palpatine truly believed his master to be incredibly wise and powerful back when they were a genuine partnership in the dark side, long before Darth Sidious came to believe that he'd finally sured his master's long-held power and wisdom as the galaxy's new Sith master.