On the positive side that words change meaning over time, it has been noted that languages that stay alive, adapt and grow over time.
An example would be the word nice. Nice used to be an insult and meant foolish or stupid in the 13th century and it went through many changes right through to the 18th century with meanings like wanton, extravagant, elegant, strange, modest, thin, and shy or coy. Now it means a good & pleasing or thoughtful & kind. Silly meant blessed or happy in the 11th century and went through pious, innocent, harmless, pitiable and feeble minded before ending up as foolish or stupid. Pretty started as crafty this changed to clever or skillfully made, then to fine and ended up as beautiful.
There are several reasons for words change meaning. One is the influence of other languages and cultures. Throughout history, many nations through conquering or intermixing with one another, introduced their own languages into the mix. Another reason is the predominate use of slang words. We get so used to using them that many times we forget that we even are.
So when it comes to defining words, there is the standard dictionary, which by the way can show multiple meanings for one word, and there is the definition of the word in slang.
As well, slang words spread faster and are used more often than before, making them common place in many societies. Hope this helped!!
