This K-pop slang term refers to smaller, up and coming, or unknown idol groups in a negative way. It literally translates from Korean (누구) to English as “Who?” To get a feeling for it, an English exchange would go something like this:
“Have you heard of The Band CAMINO?” she asked.
“Who?” he asked sarcastically.
The girl groups Weki Meki or tripleS could be described as nugu, regardless of quality.