(The Meanings of Words Should Not be Allowed to Vary or Change, Language Myths, Peter Trudgill)
All languages change all the time. It is not very well understood why this is the case, but it is a universal characteristic of human languages. The only languages which do not change are those, like Latin, which nobody speaks. Language changes their pronunciations through time. Five hundred years ago, all English speakers used to pronounce the k in 'knee' – now nobody does. Grammatical structures also change.
English speakers used to say 'Saw you my son?' Now everybody says 'Did you see my son?' But perhaps the most obvious way in which languages change is in the usage and meaning of words.
Actually, the history of the word 'nice' provides a very good illustration
of the untenable nature of this way of thinking. 'Nice' comes originally from two ancient Indo-European roots, '*skei' meaning 'cut', which came down into Latin as the verb 'scire' 'to know', probably via a meaning such as 'be able to distinguish one thing from another', and '*ne' meaning 'not'. The combination of the two forms gave the Latin verb 'nescire' which meant 'to be ignorant of'. This led to the
development of the adjective 'nescius' 'ignorant', which came down into Old French as 'nice' meaning 'silly'. It was then borrowed from French into medieval English with the meaning 'foolish, shy' and, over the centuries, has gradually changed its meaning to 'modest', then 'delicate', 'considerate', 'pleasant' and finally 'agreeable' — a very long way in 6,000 years from its original meaning. No one in their right mind, though, would argue that the 'real' meaning of nice is, or ought to be, 'not cutting'.
(...) none of us can unilaterally decide what a word means. Meaning of words are shared between people — they are a kind of social contract we all agree to — otherwise communication would not be possible.
Words do not mean what we as individuals might wish them to mean, but what speakers of the language in general want them to mean. These meanings can and do change as they are modified and negotiated in milions of everyday exchanges over the years between one speaker and another. Language change cannot be halted. Nor should the worriers feel obliged to try to halt it. Languages are self-regulating systems which
can be left to take care of themselves. They are self-regulating because their speakers want to understand each other and be understood.