Taking a Sight
Putting the right thumb to the nose and spreading the fingers out. This is done as much as to say, “Do you see any green in my eye?” “Tell that to the marines;” “Credat Judaeus, non ego.” Captain Marryat tells us that some “of the old coins of Denmark represent Thor with his thumb to his nose, and his four fingers extended in the air;” and Panurge (says Rabelais, Pantagruel, book ii. 19) “suddenly lifted his right hand, put his thumb to his nose, and spread his fingers straight out” to express incredulity.
The sacristan he says no word that indicates a doubt, But puts his thumb unto his nose, and spreads his fingers out.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894