1.
If someone is dressed up , they are wearing special clothes, in order to look smarter than usual or in order to disguise themselves.
You’re all dressed up. Are you going somewhere?...
ADJ : usu v-link ADJ
2.
If you say that something is dressed up as something else, you mean that someone has tried to make it more acceptable or attractive by making it seem like that other thing.
He tried to organise things so that the trip would be dressed up as a UN mission.
ADJ : v-link ADJ as/in n [ disapproval ]
3.
dressed up to the nines: see nine