BSD

BSD is an acronym for Berkeley Software Distribution, a flavor of Unix operating system that reigned in the 1970s and 1980s. Today, the term BSD (or *BSD) refers to one of its modern derivatives: usually FreeBSD, NetBSD or OpenBSD. To this day, NetBSD continues to custom of including Rogue, Hack and Larn in the system.

You can run NetHack on BSD, but the situation has changed so much since then that some will "#define SYSV" and "/* #define BSD */" in config.h.