Use the noun frock as an old-fashioned way to say "dress." You might wear a new pink frock to your best friend's birthday party.
Typically, girls and women wear frocks, especially to formal events like weddings and fancy parties. The word frock isn't as common today as it was in the past, though it's a great way to refer to a dress. You can also call a monk's loose, long-sleeved garment a frock. The word's origin is Germanic, and it comes directly from the French word froc, "a monk's habit."