<jargon> Reserved for exclusive use by something. The term might mean only writable by whatever it is sacred to.
For example, "Register 7 is sacred to the interrupt handler" would mean that if any other code changed the contents of register 7, dire consequences would ensue.
Try this search on Wikipedia, OneLook, Google
Nearby terms: SAC-1 « SAC2 « sacadm « sacred » SAD » SAD SAM » SADT