What is "dequeismo"

Dequeísmo is a phenomenon in Spanish grammar, considered "wrong" in prescriptive works. It is using de que instead of que as the complementizer introducing a verbal complement clause. It can be seen as the opposite of queísmo, which involves using que when de que is to be used.

For example, Me dijo estaba cansado ("He told me that he was tired"), is a case of dequeísmo since the prescriptive construction is Me dijo estaba cansado. Dequeísmo is considered peculiar to less educated speakers, most likely as an instance of hypercorrection in the attempt to avoid queísmo or perhaps a conflation with the reflexive use, which can sometimes be quite superficially similar:

Me alegr de que seáis felices. (Grammatically normal, with a reflexive verb: "I am pleased...") Me alegr de que seáis felices. (Prescriptively incorrect, non-reflexive dequeísmo, usually Me alegra que seáis felices. "It pleases me...")