The first group of causes is traditionally termed historical or extra-linguistic.
Different kinds of changes in a nation's social life, in its culture, knowledge, technology, arts lead to gaps appearing in the vocabulary which beg to be filled. Newly created objects, new concepts and phenomena must be named. We already know of two ways for providing new names for newly created concepts:
making new words (word-building) and borrowing foreign ones. One more way of filling such vocabulary gaps is by applying some old word to a new object or notion.
When the first textile factories appeared in England, the old word mill was applied to these early industrial enterprises. In this way, mill (a Latin borrowing of the first century В. С.) added a new meaning to its former meaning "a building in which corn is ground into flour". The new meaning was "textile factory".
A similar case is the word carriage which had (and still has) the meaning "a vehicle drawn by horses", but, with the first appearance of railways in England, it received a new meaning, that of "a railway car".
The history of English nouns describing different parts of a theatre may also serve as a good illustration of how well-established words can be used to denote newly-created objects and phenomena. The words stalls, box, pit, circle had existed for a long time before the first theatres appeared in England. With their appearance, the gaps in the vocabulary were easily filled by these widely used words which, as a result, developed new meanings.1
New meanings can also be developed due to linguistic factors (the second group of causes).
Linguistically speaking, the development of new meanings, and also a complete change of meaning, may be caused through the influence of other words, mostly of synonyms.1
Let us consider the following examples.
The Old English verb steorfan meant "to perish". When the verb to die was borrowed from the Scandinavian, these two synonyms, which were very close in their meaning, collided, and, as a result, to starve gradually changed into its present meaning: "to die (or suffer) from hunger".
The history of the noun deer is essentially the same. In Old English (O. E. deor) it had a general meaning denoting any beast. In that meaning it collided with the borrowed word animal and changed its meaning to the modern one ("a certain kind of beast", R. олень).
The noun knave (O. E. knafa) suffered an even more striking change of meaning as a result of collision with its synonym boy. Now it has a pronounced negative evaluative connotation and means "swindler, scoundrel".