Origins
Sacred contains a paradox inherited from Latin. The ancestor word sacer meant both 'holy' and 'accursed'. This was not a contradiction — it was a description. Something sacer was set apart from the ordinary world, removed from human use. Whether it was set apart for honour or for punishment was secondary to the act of separation itself.
The Roman concept of homo sacer illustrates this perfectly. A person declared sacer could be killed by anyone without legal consequence, yet could not be sacrificed to the gods. They existed outside both human law and divine ritual — set apart in the fullest sense.
From sacer came sacrāre ('to make sacred'), sacrifice ('to make something sacred by offering it'), sacrament ('a sacred act'), and sacrilege ('theft from a sacred place'). Saint comes from the related Latin sanctus — also meaning 'set apart, holy'.
Middle English
The word entered English via Old French sacré in the 14th century, by which time the 'accursed' sense had faded. Modern English sacred carries only the positive meaning. But French preserves a trace of the duality: sacré is used as both a reverent term and a mild expletive — 'sacré bleu' being the famous example.
In secular use, sacred now means 'too important to tamper with'. A sacred tradition, a sacred right, a sacred space. The religious origin has loosened, but the core idea remains: that which is set apart.