Neomu lonely is a popular catchphrase by a famous K-pop idol, Lucas NCT. This catchphrase usually use by people who was having a trainee period but at the same time, your friend named Mark Lee debuted first and debuted again and again.
Example:
Lucas: I trained with Mark.
Mark: WHEEZE
Lucas: but he debuted first
Mark: WHEEZE
Lucas: neomu lonely
Mark: WHEEZE