Pretty is a very interesting word beginning with how it is pronounced.
Why is the ‘e’ pronounced as /i/ like in insect and not /e/ like in egg?
It is actually not uncommon for the ‘e’ to represent the /i/ sound. Think, for example, about how you pronounce ‘e’ in the following words in everyday conversation – England, English, knowledge, privilege.
All of the vowels in English can be pronounced in multiple ways (e.g., ‘a’ can be pronounced as /a/-apple, /ay/-table, /ar/-bath, /i/-marriage, /or/-ball, talk, /u/-banana, /e/-many, /o/, watch). So, the fact that ‘e’ can be pronounced as /i/ is not so strange. In fact, ‘e’ can also be pronounced as /o/ like in entrée in addition to /e/-egg and /ee/-me.
The other issue is that the pronunciation of words change over time. If you look at the history of ‘pretty’, it appears to have been pronounced as ‘prættig’ (i.e., prattig) in Old English and it is believed to have come from the Dutch ‘pret’ (pronounced as it is written). A lot of changes in the pronunciation of vowels occurred during the ‘great vowel shift’ where the pronunciation of vowels changed but the spelling stayed the same often resulting in a mismatch between the written and verbal forms.
The dictionary definition of pretty is along the lines of ‘a person, especially a woman or child, who is attractive in a delicate way without being truly beautiful’ or ‘an attractive thing, especially a trinket’. It can also mean ‘quite’ (e.g., He was pretty fit for his age) or used ironically to express annoyance of displeasure (e.g., He got us into a pretty mess).
However, interestingly, in Old English ‘pretty’ had the meaning of tricky or wily. It then apparently disappeared during the Middle English period and reappeared in the 15th century where it was used to mean ‘clever’ or ‘skilful’.
From the 1400s onward, pretty came to mean ‘elegantly made or done’ (like a pretty speech) and then was subsequently used to describe things, places, and people that were aesthetically pleasing, similar to how it is currently used. Although today we tend to use ‘pretty’ in reference to women and children, for a while it was used to describe men as either ‘good looking’ (a pretty lad) or as ‘brave, gallant and warlike’.
By the 1700s, the word ‘pretty’ was only used in reference to men who were seen as overly concerned with their appearance and for women it was often used in a derogative sense in that a woman was either pretty OR intelligent (e.g., Don’t worry your pretty head about those accounts).
Language is continually involving. I wonder how we will be using the word ‘pretty’ next century??
References
https://www.byrdie.com/history-word-pretty
https://www.lexico.com/en/definition/pretty
https://www.etymonline.com/search?q=pretty