Þanon untȳdras ealle onwōcon,
eotenas ond ylfe ond orcnēas,
swylce gigantas, þā wið Gode wunnon
ogres and elves and evil phantoms
and the giants too who strove with God
It was probably borrowed from Latin Orcus "the lower world, the abode of the dead". Ogre is possibly from this word as well, thru French. Also Italian orco "demon, monster", Spanish huerco "devil", Sardinian orcu "demon", and early modern Dutch orck "unruly person".
The Proto-Indo-European root is *areq- "to guard, lock". I don't know what that q is supposed to be. This became another Latin word, arcānus "secret" and then arcane.