It is common in informal speech and writing but is neither rare nor wrong in serious discourse See ‘meaning & use’ for definitions, usage, and quotation evidence. We can use pretty as an adverb, before an adjective or another adverb, meaning ‘quite, but not extremely’
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… that's a pretty hat you're wearing
The sofa was covered in very pretty flowery material
She's got such a pretty daughter She looked pretty in a simple cotton dress. Pleasing or attractive to the eye, as by delicacy or gracefulness. See examples of pretty used in a sentence.
Find 448 different ways to say pretty, along with antonyms, related words, and example sentences at thesaurus.com. If you describe someone as pretty, you mean that they are attractive She's a very charming and very pretty girl. Use the word pretty to describe something with a delicate and pleasant appearance
Something that’s pretty is less powerful and intimidating than something that’s beautiful, the way a pretty floral raincoat is less intimidating than a beautiful film star.
When particularly stressed, the adverb pretty serves almost to diminish the adjective or adverb that it modifies, by emphasizing that there are greater levels of intensity. There are 20 meanings listed in oed's entry for the word pretty, four of which are labelled obsolete
