Sesquipedalian bindings are bindings that stop a person from pronouncing a certain word. In a djinn's case, this can be very dangerous because it can stop them from uttering their focus word, which is part of the reasons why it is supposed to be kept secret.
Nimrod cast one of these on both John and Philippa to prevent them from changing their accomidations at the El Moania, which he arragned to "educate" them on the was of the other walk of life. He soon lifted them after they arrived at Mr. Barkhiya's flying carpet shop so they could properly admire the power contained in the fibers of King Solomon's Carpet.
The Sesquipedalian binding comes from the same word as Mr. Rakshasas's focus word SESQUIPEDALIAN, meaning a word that is very long. The binding could have orginally been invented to prevent the djinn from saying all long words, including their focus words.