Green’s Dictionary of Slang

1. (UK juv.) an aggressive person, a ‘hard’ man.

1927		F.M. Thrasher Gang 340: ‘A hard boy [...] Nails’.
1989		P. Dickson Baseball Dict. (1999) 336: Nails [...] Generic nickname for a player who is tough.
2001		OnLine Dict. of Playground Sl. 🌐 nails n. hard case. someone who is fit and good at fighting, tough guy.

2. see nail n.¹ (1)