IPA: //ˈdrɛs.i.ər//
KK: /ˈdrɛsɪər/
More formal or stylish than usual clothing.
For the party, she chose a dressier outfit than she normally wears.
Dressier → It is formed from "dress" (from Old French *drecier*, meaning to arrange or put in order) and "-ier" (a suffix indicating a person or thing associated with a particular quality). The word "dressier" refers to someone or something that is more arranged or put in order, particularly in terms of clothing or appearance.
Think of someone who is 'arranging' ('drecier') their clothing to look 'more' stylish — that's what dressier means.