_Sad_ in Old English meant to have as much as one wanted of anything. Then it came to mean "calm" and "serious," perhaps from the idea that people who have all they want are in a mood to settle down and attend to things seriously. Already in Shakespeare's writings we find the word with its present meaning of "sorrowful." It has quite lost its earlier meaning, but has several special new meanings besides the general one of "sorrowful." A "sad tint," or colour, is one which is dull. "Sad bread" in the north of England is "heavy" bread which has not risen properly. Again, we describe as "sad" some people who are not at all sorrowful. We say a person is a "sad" liar when we mean that he is a hopeless liar.
The word _tide_, which we now apply to the regular rise and fall of the sea, used to mean in Old English "time;" and it still keeps this meaning in the words _Christmastide_, _Whitsuntide_, etc.
One common way in which words change is in going from a general to a more special meaning. Thus in Old English the word _chest_ meant "box" in general, but has come now to be used as the name of a special kind of box only, and also as the name of a part of the body. The first person who used the word in this sense must have thought of the "chest" as a box containing the lungs and the heart.
_Glass_ is, of course, the name of the substance out of which we make our windows and some of our drinking vessels, etc., and this was at one time its only use; but we now use the name _glass_ for several special articles--for example, a drinking-vessel, a telescope, a barometer, a mirror (or "looking-glass"), and so on. _Copper_ is another word the meaning of which has become specialized in this way as time has gone on. From being merely the name of a metal it has come to be used for a copper coin and for a large cauldron especially used in laundry work. Another example of a rather different kind of this "specialization" which changes the meaning of words is the word _congregation_. _Congregation_ used to mean "any gathering together of people in one place," and we still use the word _congregate_ in this sense. Thus we might say "the people congregated in Trafalgar Square," but we should never think of speaking of a crowd listening to a lecturer there as a "congregation." The word has now come to mean an assembly for religious worship in a chapel or church.