The “genius” that pluralises to “genii” refers to a guardian/influential spirit, not a person of exceptional mental capacity. Please, Colfer, learn to use a dictionary.
Actually, "genii" is an accepted pluralization of "genius."
In Latin, about 99% of all words have a long series of endings called declensions that indicate their exact function in the sentence. Because of these declensions, the plural of "alumna" is "alumnae," the plural of "datum" is "data," the plural of "opus" is "opera," and the plural of "alumnus" is "alumni" (pronounced the same as "alumnae," actually).
"Genius" would logically follow the pluralization pattern of "alumnus" ("opus" belongs to an entirely different set of declensions that also change the form of the word). Therefore, "genius" can be acceptably pluralized to "genii." Even if this is the lesser-used spelling, and even if it is other definitions in English, Artemis (and Minerva) would still use it--I think that it would be completely in character. Besides, "genii" sounds more erudite than "geniuses." Obviously, Artemis knows his Latin, certainly better than I do (which is bad, because my AP Latin exam is in 3 weeks).
Sorry if this is a bit verbose, but I felt the need to defend Colfer (since he is usually such a poor self-editor).
In Defense of Colfer
Actually, "genii" is an accepted pluralization of "genius."
In Latin, about 99% of all words have a long series of endings called declensions that indicate their exact function in the sentence. Because of these declensions, the plural of "alumna" is "alumnae," the plural of "datum" is "data," the plural of "opus" is "opera," and the plural of "alumnus" is "alumni" (pronounced the same as "alumnae," actually).
"Genius" would logically follow the pluralization pattern of "alumnus" ("opus" belongs to an entirely different set of declensions that also change the form of the word). Therefore, "genius" can be acceptably pluralized to "genii." Even if this is the lesser-used spelling, and even if it is other definitions in English, Artemis (and Minerva) would still use it--I think that it would be completely in character. Besides, "genii" sounds more erudite than "geniuses." Obviously, Artemis knows his Latin, certainly better than I do (which is bad, because my AP Latin exam is in 3 weeks).
Sorry if this is a bit verbose, but I felt the need to defend Colfer (since he is usually such a poor self-editor).