E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Sir W. Scott calls Jack oLantern Friar Rush. This is an error, as Rush was a domestic spirit, and not a field esprit follet. He got admittance into monasteries, and played the monks sad pranks, but is never called Jack. Sir Walter Scott seems to have considered Friar Rush the same as Friar with the Rush (light), and, therefore, Friar with the Lantern or Will o the Wisp.
Better we had through mire and bush
Been lanthorn-led by Friar Rush.
Sir Walter Scott: Marmion.
Milton also (in his LAllegro) calls Will o the Wisp a friar, probably meaning Friar Rush:
She was pinched, and pulled, she said;
And he by Friars lantern led.
but Rush in this name has nothing to do with the verb rush [about] or rush [light]. It is the German Brüder Rausch, called by the Scandinavians Broder Ruus. (Scandinavian, ruus, intoxication, in German rausch, which shows us at once that Friar Rush was the spirit of inebriety. (See ROBIN GOODFELLOW.)