What's in a name? That which we call a rose, By any other name would smell as sweet.
“Thou shalt not take the name of the Lord thy God in vain.” - Exod. xx. 7.
Fairies are extremely averse to having their names known, indeed there seems to be a strange identity between personality and name. Thus we are forbidden to take God's “name in vain,” and when Jacob wrestled with the angel, he was anxious to know his opponent's name. (Compare the Greek onoma and the Latin anima.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894