A word that happends to have a very different denotation and connotation. (In other words, a word that has a "definate definition" but a different "feel" to it.)
However it is not always perceived that way, because of the negative contexts that it is used in.
For example, (1) movies that are R-rated or worse and often have no censorship are considered "unadulterated" bc said movies are not "blocked/filtered/diluted" by the censors. The disgusting movies are considered "unadulterated," "pure," and/or "pure unadulterated fun." Thus, the word "unadulterated" becomes associated with dirty movies and/or dirty things.
Similarly, (2) the term "unadulterated jealousy" technically means "pure jealousy" and/or "just jealousy." However the term "feels" like: extreme jealousy, jealousy-without-restraint, and/or unhinged jealousy.
Because of the often repetitive use of "unadulterated" in negative contexts (especially in slang and/or common spoken language), it starts-to-sound-like a bad word. The word "unadulterated" grows a negative connotation! The word no longer has a "pure" nor "NOT-diluted" feel to it.
(The word has a neutral denotation but a negative connotation.) The word has a new definition to it: bad, tainted, without restraint, dirty, unhinged.