It is a slang. 급 실망(sudden disappointment) actually does not exist in Korean dictionary but lately people like to add 급 in front of any kind of noun to express the event happened all of sudden. 급 is originally a root of the word 급하다(urgent). it is only used as an element of nouns such as 급체(acute indigestion), 급변(a sudden change), adjectives such as 급격한(rapid) or the like.