To put on special or fancy clothes.
Everyone dressed up for the graduation ball.
(often with 'as') To put on a costume portraying oneself as a particular type of character or well-known person.
We're having a party on Saturday, but you must dress up as a famous historical figure.
To decorate; to prettify.
To present in a favorable light.
You can dress up that proposal however you want, it's still going to go over like a lead balloon.