(With a), easily, directly. “D'un tour de main. ”The allusion is to the old custom of spinning, in which the spinner constantly wetted the fore-finger with the mouth.
“I can bring myself round with a wet finger.”
“The spirit being grieved and provoked. ... will not
return again with a wet finger.” —
I can find One with a wet finger that is stark blind.
Trial of Love and Fortune (1598). Flores. “Canst thou bring me thither? Peasant. With a wet finger.”
Wisdom of Dr. Dodypoll (1600).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894