What do we call the process where a word's meaning becomes more negative over time?

The correct term for the process where a word's meaning becomes more negative over time is known as "deterioration." This phenomenon occurs when the connotation of a word shifts from positive or neutral to negative, often reflecting societal attitudes and perceptions about the referent of the word. An example of this is the word "silly," which once meant happy or fortunate but has come to imply foolishness or lack of good sense.

The other options do not accurately describe this specific process. "Elevation" refers to the opposite process, where a word's meaning becomes more positive. "Neologism" pertains to the creation of new words or phrases, unrelated to changes in meaning of existing words. "Sound reduction" is a phonetic process where sounds in a word are omitted or lessened, and it does not address meaning changes at all. Thus, "deterioration" is the most appropriate term for the negative shift in meaning.