From the Latin "caveo", meaning "on guard against", and "inimucus", meaning "hostile".
This spell was not fulling explained, but Harry, Rom and Hermione had used it in the Deathly Hallows as a means of protecting themselves against the Death Eaters. Hermione had waved her wand at the sky as she spoke the incantation. The name suggests that it prevents hostile magic from entering the protected area.
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