“Words are created.
Often they spring seemingly from nowhere. Take “dog”. For centuries the word in English was “hound” (or “hund”). Then suddenly in the late Middle Ages, “dog” – a word etymologically unrelated to any other known word – displaced it. No one has any idea why.
Many words are made up by writers. Consider the words that Shakespeare alone gave us: obscene, frugal, radiance, excellent, fretful, hint, hurry, lonely and some 1700 others.
Words change by doing nothing
That is, the word stays the same but the meaning changes.
Simeon Potter notes that when James II saw St Paul Cathedral he called it amusing, awful and artificial and meant that it was pleasing to look at, deserving of awe and full of skilful artifice.
Sometimes the changing connotations of a word can give a new and startling sense to literary passages, as in “The Mayor of Casterbridge” where Thomas Hardy has one of his characters gaze upon”the unattractive exterior of Farfrae’s errection” or in “Bleak House” where Dickens writes that “Sir Leicester leans back in his chair and breathlessly ejaculates”.
Occasionally, because the sense of the word has changed, fossil expressions are misleading. Consider the oft-quoted statement “the exception proves the rule”. Most people take this to mean that the exception confirms the rule, though when you ask them to explain the logic in that statement, they usually cannot. After all, how can an exception prove a rule? It can’t. The answer is that an earlier meaning of “prove” was “to test” and with that meaning the statement suddenly becomes sensible – the exception tests the rule.
Words are created by adding or substracting something
This inclination to use affixes and infixes provides gratifying flexibility in creating or modifying words to fit new uses, as strikingly demonstrated in the word “incomprehensibility” which consists of the root “-hen-“ and eight affixes and infixes: in, -com, -pre-, -s-, -ib-, -il-, -it-, -y. Even more melodic is the musical term “quasihemidemisemiquaver”, which describes a note that is equal to 128th of a semibreve.
Many of our most common words are contractions of whole phrases – for instance, “goodbye”, a shortening of “God be with you” and “hello”, which was in Old English “hal beo thu” or “whole be thou” ”
BillBryson.MotherTongue