washed up is the past form of wash up (just add -ed to the main verb).
wash up is a phrasal verb which means  to do the dishes, to wash plates, glasses,ect after the meal.
In American English, it also means to wash your face and hands.
For example, when a mother tell her son “to get washed up”, it doesn’t means she want her son to wash dishes and plates, she want her son to wash his face and hands.

Very well explained @Minh Khoi