Brewer's: Genius

Genii (Roman mythology) were attendant spirits. Everyone had two of these tutelaries from his cradle to his grave. But the Roman genii differ in many respects from the Eastern. The Persian and Indian genii had a corporeal form, which they could change at pleasure. They were not guardian or attendant spirits, but fallen angels, dwelling in Ginnistan, under the dominion of Eblis. They were naturally hostile to man, though compelled sometimes to serve them as slaves. The Roman genii were tutelary spirits, very similar to the guardian angels spoken of in Scripture (St. Matt. xviii. 10). (The word is the old Latin geno, to be born, from the notion that birth and life were due to these dii genitales.)

Genius

(birth-wit) is innate talent; hence propensity, nature, inner man. “Cras genium mero curabis” (to-morrow you shall indulge your inner man with wine), Horace, 3 Odes, xvii. 14. “Indulgere genio” (to give loose to one's propensity), Persius, v. 151. “Defraudare genium suum” (to stint one's appetite, to deny one's self), Terence: Phormio, i. 1. (See above.)

Genius.

Tom Moore says that Common Sense went out one moonlight night with Genius on his rambles; Common Sense went on many wise things saying, but Genius went gazing at the stars, and fell into a river. This is told of Thale by Plato, and Chaucer has introduced it into his Milleres Tale.

So ferde another clerk with astronomye: He walkëd in the feeldës for to prye Upon the sterrës, what ther shuld befall, Till he was in a marlë pit i-fall.

Canterbury Tales, 3,457.

My evil genius

(my ill-luck). The Romans maintained that two genii attended every man from birth to death—one good and the other evil. Good luck was brought about by the agency of “his good genius,” and ill luck by that of his “evil genius.”

Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894

Genius Loci

Genitive Case