what is it called when you use two words with contradictory meanings for example: that girl is pretty ugly
In the sentence in question pretty is not used as an adjective meaning attractive. Instead, it's an adverb meaning somewhat; to a certain degree.
1. To a fair degree; moderately: a pretty good student.
pretty - definition of pretty by the Free Online Dictionary, Thesaurus and Encyclopedia.
I've seen 'airline food' on many such lists and, while amusing, it is a very dubious one.