Originally, Semantics meant historical semantics — meaning change over time. All languages change, sometimes very quickly. Sometimes the meaning of words change. This is the focus of Historical Semantics.
Etymology is an important part of Historical Semantics; it is the study of the origins and histories of words. You can find historical information about the origin of a word in ordinary dictionaries as well as in specialized etymological dictionaries.
Etymologists are concerned with where words (and related phrases) came from originally and how meaning has changed over time. The word etymology itself comes from the Greek word etymon which means “the true meaning of a word.” How did the word etymon enter the Greek language? Good question. Usually, however, the further back we look, the less evidence we can find. Very often, etymologists make guesses about the origins of words.
Consider, for example, the English words cook and cookie. While these words look similar, they are not related etymologically, as far as we can tell. The word cook, meaning “someone who cooks” moved from Latin into Old English while cookie, meaning a kind of biscuit, originated in Dutch and meant “a little cake.” Of course, if you try to look at the origins of words further and further back in time, evidence gets harder to find.
Words often acquire different meanings as time passes. There might be a change in the concepts associated with a word, for example. Alternatively, the meaning of one word might split into two or more concepts. It is also possible for two separate concepts to merge in one word.
Sometimes a word might get a wider or narrower meaning than previously. For example, the modern English word bird comes from the Old English bridd, which meant “young bird.” The general word meaning “bird” in Old English was fugol, which corresponds to the modern word fowl. The modern English word meat originally meant “food in general.”
Words often change as they take on new metaphorical meaning. For example, the modern English word field originally meant “a piece of land” but now also means “an area of scientific study.”
Metonymy is when the name of one thing replaces that of something else, usually something closely related. For example, tea has come to mean “afternoon meal” as well as the famous drink.
Also, the crown, the hat worn by a king or queen, came to mean political power or authority.
Sometimes part of a word’s meaning, its connotations, can change. For example, the connotation of the modern word cunning is strongly negative today, meaning something like “sneaky.” However, in Old English the word had the strongly positive connotation of “skillful or expert.”
Sometimes completely new words enter a language. We call this linguistic borrowing. Many loan-words entered the English language from French after the Norman Conquest in 1066. Japanese has many loan-words from English.
Sometimes new words are created by analogy. A famous example is hamburger. This originated through a combination of the German town Hamburg and the affix -er. In other words, the famous meat dish was a Hamburg#er. English speakers understood this differently, however. I always thought the original Ham#burger was made of ham and I always thought it was strange that I had never eaten the original ham#burger. I had only ever eaten beef#burgers. I wasn’t the only person to think like this, and later I discovered fish#burgers, cheese#burgers, chili#burgers etc.
Clearly, there are lots of different reasons behind lexical change.