Mispronouncing words, using incorrect grammar, and misusing words when speaking are all examples of cacology. In the world of linguistics, cacology is the antithesis of eloquence and proper diction.
The first part of cacology, caco, is derived from the Greek kakos, meaning "bad." That root is also seen in the word cacophony, in which it combines with the root phon-, meaning "sound," to describe disagreeable, unharmonious — i.e., bad-sounding — noise. When caco is combined with logy, derived from the Greek logia-, meaning "speech, word, or study," we get cacology, or literally "bad speech." It's best to avoid cacology when giving a formal speech or presentation.