Green’s Dictionary of Slang

(US) the Mafia.

1984		W. Diehl Hooligans (2003) 36: ‘What’s the LCN?’ Zapata asked. ‘La Costra Nostra, you fuckin’ moron,’ Salvatore taunted.
2000		Lehr & O’Neill Black Mass 10: Zannino was the brutal and bloodless mafioso whom Angiulo relied on to bring muscle to the Boston LCN enterprise.