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