In historical linguistics, it refers to the any change in the meaning(s) of a word through the development of languages over the course of time.
Also called semantic shift, lexical change and semantic progression.
"Semantic change is not a change in meaning per se, but the addition of a meaning to the semantic system or the loss of a meaning from the semantic system while the form remains constant." --David P. Wilkins
The word is from Old English "prættig" which means "cunning"
By the 15th century, people used it to describe something that is made cleverly or artful.
Over time, the modern use for the word pretty is not used to describe someone who is attractive and good-looking or pleasing to the eyes. The word is often thrown as a compliment, mostly towards females.
There are, however, ironic ways to use "pretty". Phrases like "pretty pass" or "pretty state of affairs"