Nugu (누구) is a Korean word that means “who”. In the context of K-pop, it refers to groups that aren’t popular enough (if at all), usually due to being managed by smaller, less notorious labels. The English equivalent would be something akin to calling a group “a bunch of nobodies”.

Since the K-pop industry has been growing in the past years, the competition among companies has become even more fierce, which in turn has led to a proportional increase in the number of attempts that are ultimately labeled as nugu groups.

It isn't unknown for a nugu group to break out and finally reach stardom (an example would be EXID's recent rise to popularity due to the success of a particular fancam), but it is still considered highly unlikely.

Context: "Everyone always talks about the really talented underrated groups but what are some "nugu " who should stay "nugu" ?"

Context reference: http://www.allkpop.com/forums/discussion/197520/nugu-groups-who-will-always-be-nugu

Usage comment: although this entry is listed as a noun, this term may also be used as an adjective.

Definition reference: Urban Dictionary (http://pt.urbandictionary.com/define.php?term=Nugu)

Wiktionary (https://en.wiktionary.org/wiki/%EB%88%84%EA%B5%AC)