What is the word for when you join two opposites like pretty ugly?

The word for when you join two opposites like pretty ugly is an oxymoron. An oxymoron is a figure of speech that juxtaposes two contradictory terms. For example, "pretty ugly" is an oxymoron because "pretty" and "ugly" are opposites. Other examples of oxymorons include "jumbo shrimp," "deafening silence," and "bittersweet."